move declaration of cpu_set_t to ext_hotprofiler.h to fix OS X build

Summary: Fixes issue https://github.com/facebook/hhvm/issues/3079

cpu_set_t is used in ext_hotprofiler.h (line 18 of .cpp, line 129 of ext_hotprofiler.h) before it has been defined in ext_hotprofiler.cpp (at line 51).
Closes #3106

Reviewed By: @ptarjan

Differential Revision: D1422472

Pulled By: @fredemmott