src: missing header for std::function
src: missing header for std::function

gcc-c++ 7.1.1 fails build without this include:
```
./lightstep/options.h:42:8: error: 'function' in namespace 'std' does not name a template type
   std::function<uint64_t()> guid_generator;
        ^~~~~~~~
```

See related issue: https://github.com/lyft/envoy/issues/1387

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp,lightstep/lightstep-tracer-cpp