[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
diff --git a/src/filesystem/posix_compat.h b/src/filesystem/posix_compat.h
index 8062bd6..f5c8ec3 100644
--- a/src/filesystem/posix_compat.h
+++ b/src/filesystem/posix_compat.h
@@ -487,7 +487,9 @@
 using ::chdir;
 using ::close;
 using ::fchmod;
+#if defined(AT_SYMLINK_NOFOLLOW) && defined(AT_FDCWD)
 using ::fchmodat;
 using ::fstat;
 using ::ftruncate;
 using ::getcwd;