[libc++] Fix preprocessor guard for overload declaration
[libc++] Fix preprocessor guard for overload declaration

Fix a typo in the conditional.  Caught by going through list of removed
symbols when building with hidden visibility.

Differential Revision: https://reviews.llvm.org/D26825

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@287309 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx