[libcxx] Suppress unused warning on apple.

Summary:
This warning is already suppressed on non-apple platforms, so
this change just suppresses it on apple as well.

Reviewers: EricWF, lichray

Reviewed By: lichray

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41248

llvm-svn: 321435

libcxx/src/experimental/filesystem/operations.cpp

@@ -512,6 +512,7 @@ using StatT =  struct stat;
 
 #if defined(__APPLE__)
 TimeSpec extract_mtime(StatT const& st) { return st.st_mtimespec; }
+__attribute__((unused)) // Suppress warning
 TimeSpec extract_atime(StatT const& st) { return st.st_atimespec; }
 #else
 TimeSpec extract_mtime(StatT const& st) { return st.st_mtim; }