Don't #include libelf on OS X

Summary: We don't need libelf includes on OS X here; we have our own
implementation on Mach-O. Actually including the headers is annoying --
for compatibility reasons, OS X package managers to build libelf in
"disable compat" mode, which doesn't install these headers in
/usr/include, so you need to do `-I/foo/bar/libelf` -- but `libelf.h`
itself does a `#include <libelf/libelf.h>` so you //also// need
`-I/foo/bar` if `/foo/bar` isn't a standard include directory (again
common on OS X).

But this whole thing should be moot, we don't need it, just move it
under ifdef.

Reviewed By: @ptarjan

Differential Revision: D1482087