[SystemZ][ZOS] Guard using declaration for ::fchmodat

The use of fchmodat() is beeing guarded but its using declaration is not. Let's use the same guard in both places to avoid compiler errors on platforms where `fchmodat` does not exist.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D96303

GitOrigin-RevId: 25aa0d12445eed6e278489546d18fcb7a33cfaa6
1 file changed