Don't try to call _free_osfhnd when not compiling agains the static CRT

Don't try to call _free_osfhnd when not compiling agains the static CRT

Summary:
Because, unfortunately, it isn't exported from the dynamic CRT dlls :(
There's not really a nice way to handle this when using the dynamic CRT without doing very fragile hocus-pocus that relies on the exact layout and implementation details of the file descriptor table in the CRT, so the best we can really do is close the socket and just let the file descriptor itself leak.

Reviewed By: yfeldblum

Differential Revision: D4156558

fbshipit-source-id: 32cb4bf357f6746cf6597b66649ff9f018fb1bed