Update the sys/syscall.h portability header for Windows

Update the sys/syscall.h portability header for Windows

Summary: This adds Windows support to the portability header for sys/syscall.h, which was previously named portability/Syscall.h, which is inconsistent with the naming of the other headers, so it's now also been renamed to SysSyscall.h, and the one place that used it updated to reflect the new name.

Reviewed By: yfeldblum

Differential Revision: D2984383

fb-gh-sync-id: c0e4ce55c7bc2aa6db9b084e09d9762ba587002d
fbshipit-source-id: c0e4ce55c7bc2aa6db9b084e09d9762ba587002d