Stop suppressing warnings with -Wno-cast-function-type

Without -Wno-cast-function-type, we see the following warnings when compiling with gcc 8:

```
gyp info spawn args [ 'BUILDTYPE=Release', '-C', 'build' ]
make: Entering directory '/tmpfs/src/github/pprof-nodejs/build'
CXX(target) Release/obj.target/pprof/bindings/profiler.o
In file included from /root/.node-gyp/6.0.0/include/node/v8-profiler.h:9,
from ../bindings/profiler.cc:17:
/root/.node-gyp/6.0.0/include/node/v8.h: In static member function 'static void v8::V8::RemoveGCPrologueCallback(v8::GCCallback)':
/root/.node-gyp/6.0.0/include/node/v8.h:8504:57: warning: cast between incompatible function types from 'v8::GCCallback' {aka 'void (*)(v8::GCType, v8::GCCallbackFlags)'} to 'v8::Isolate::GCCallback' {aka 'void (*)(v8::Isolate*, v8::GCType, v8::GCCallbackFlags)'} [-Wcast-function-type]
reinterpret_cast(callback));
^
/root/.node-gyp/6.0.0/include/node/v8.h: In static member function 'static void v8::V8::RemoveGCEpilogueCallback(v8::GCCallback)':
/root/.node-gyp/6.0.0/include/node/v8.h:8511:57: warning: cast between incompatible function types from 'v8::GCCallback' {aka 'void (*)(v8::GCType, v8::GCCallbackFlags)'} to 'v8::Isolate::GCCallback' {aka 'void (*)(v8::Isolate*, v8::GCType, v8::GCCallbackFlags)'} [-Wcast-function-type]
reinterpret_cast(callback));
^
In file included from ../bindings/profiler.cc:18:
../node_modules/nan/nan.h: In function 'void Nan::AsyncQueueWorker(Nan::AsyncWorker*)':
../node_modules/nan/nan.h:2232:62: warning: cast between incompatible function types from 'void (*)(uv_work_t*)' {aka 'void (*)(uv_work_s*)'} to 'uv_after_work_cb' {aka 'void (*)(uv_work_s*, int)'} [-Wcast-function-type]
, reinterpret_cast(AsyncExecuteComplete)
^
In file included from ../node_modules/nan/nan.h:53,
from ../bindings/profiler.cc:18:
../bindings/profiler.cc: At global scope:
/root/.node-gyp/6.0.0/include/node/node.h:438:43: warning: cast between incompatible function types from 'void (*)(Nan::ADDON_REGISTER_FUNCTION_ARGS_TYPE)' {aka 'void (*)(v8::Local)'} to 'node::addon_register_func' {aka 'void (*)(v8::Local, v8::Local, void*)'} [-Wcast-function-type]
(node::addon_register_func) (regfunc), \
^
/root/.node-gyp/6.0.0/include/node/node.h:469:3: note: in expansion of macro 'NODE_MODULE_X'
NODE_MODULE_X(modname, regfunc, NULL, 0)
^~~~~~~~~~~~~
../bindings/profiler.cc:189:1: note: in expansion of macro 'NODE_MODULE'
NODE_MODULE(google_cloud_profiler, InitAll);
^~~~~~~~~~~
In file included from /root/.node-gyp/6.0.0/include/node/v8-profiler.h:9,
from ../bindings/profiler.cc:17:
/root/.node-gyp/6.0.0/include/node/v8.h: In instantiation of 'void v8::PersistentBase::SetWeak(P*, typename v8::WeakCallbackInfo

::Callback, v8::WeakCallbackType) [with P = node::ObjectWrap; T = v8::Object; typename v8::WeakCallbackInfo

::Callback = void (*)(const v8::WeakCallbackInfo&)]':
/root/.node-gyp/6.0.0/include/node/node_object_wrap.h:64:78: required from here
/root/.node-gyp/6.0.0/include/node/v8.h:7484:16: warning: cast between incompatible function types from 'v8::WeakCallbackInfo::Callback' {aka 'void (*)(const v8::WeakCallbackInfo&)'} to 'Callback' {aka 'void (*)(const v8::WeakCallbackInfo&)'} [-Wcast-function-type]
reinterpret_cast(callback), type);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/root/.node-gyp/6.0.0/include/node/v8.h: In instantiation of 'void v8::PersistentBase::SetWeak(P*, typename v8::WeakCallbackInfo

::Callback, v8::WeakCallbackType) [with P = Nan::ObjectWrap; T = v8::Object; typename v8::WeakCallbackInfo

::Callback = void (*)(const v8::WeakCallbackInfo&)]':
../node_modules/nan/nan_object_wrap.h:66:61: required from here
/root/.node-gyp/6.0.0/include/node/v8.h:7484:16: warning: cast between incompatible function types from 'v8::WeakCallbackInfo::Callback' {aka 'void (*)(const v8::WeakCallbackInfo&)'} to 'Callback' {aka 'void (*)(const v8::WeakCallbackInfo&)'} [-Wcast-function-type]
SOLINK_MODULE(target) Release/obj.target/pprof.node
COPY Release/pprof.node
COPY /tmpfs/src/github/pprof-nodejs/build/node-v48-linux-x64-musl/pprof.node
TOUCH Release/obj.target/action_after_build.stamp
make: Leaving directory '/tmpfs/src/github/pprof-nodejs/build'
gyp info ok
```

We are going to suppress these warnings for now with -Wno-cast-function-type , but should stop using this flag in the future when these warnings are handled by V8.