`add_fbthrift_library` should add include path for prefix

Commit https://github.com/facebook/fb303/commit/64e0e3f0ec069ed415e4afa10e6a549d689d3c0e introduced includes from fb303 to fbthrift. This currently fails if one tries to build fb303 as a system library, because the include paths passed to `thrift1` does not include `/usr/include` which is where the Thrift annotations included by fb303_thrift live (installed by a prior fbthrift installation). Either `fbthrift1` should look at the prefix by default, or the CMake custom command generated by `add_fbthrift_library` should include it.