[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

libcxx/src/filesystem/posix_compat.h

@@ -487,7 +487,9 @@ int symlink_dir(const char *oldname, const char *newname) {
 using ::chdir;
 using ::close;
 using ::fchmod;
+#if defined(AT_SYMLINK_NOFOLLOW) && defined(AT_FDCWD)
 using ::fchmodat;
+#endif
 using ::fstat;
 using ::ftruncate;
 using ::getcwd;