master is broken on Android

`master` version (I tested revision c9311a44e1280853632fe2472345dd04514a2f74) fails to build on Android, with the error:
```
/Users/marat/benchmark/src/sysinfo.cc:586:21: error: use of undeclared identifier 'getloadavg'
const int nelem = getloadavg(res.data(), kMaxSamples);
^
1 error generated.
```

Repro:
```bash
git clone https://github.com/google/benchmark.git
cd benchmark
mkdir build && cd build
cmake -DCMAKE_TOOLCHAIN_FILE=/path/to/android/ndk/build/cmake/android.toolchain.cmake -DBENCHMARK_ENABLE_TESTING=OFF ..
make
```