[libc++] Add missing friend keyword

Otherwise, we're declaring a non-static member function, and that
gives errors in C++11 because of the change of semantics between
C++11 and C++14 for non-const constexpr member functions.

This was always intended to be a friend declaration.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: d9c9a74d0dc5b64c7c8496294ed962d7ce332337
diff --git a/include/iterator b/include/iterator
index 45516db..e2910e9 100644
--- a/include/iterator
+++ b/include/iterator
@@ -1618,7 +1618,7 @@
     __unwrap_iter(__wrap_iter<_Tp*>);
 #else
   template <class _Tp>
-  inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+  inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR friend
   typename enable_if
   <
       is_trivially_copy_assignable<_Tp>::value,