[libc++] Add missing visibility annotation for __base

Differential Revision: https://reviews.llvm.org/D48680

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 30ccc2e8d24b53273aae9cc21740f02ffb3018dc
diff --git a/include/functional b/include/functional
index 63e3cbe..b13992f 100644
--- a/include/functional
+++ b/include/functional
@@ -1618,7 +1618,7 @@
 
 // __base provides an abstract interface for copyable functors.
 
-template<class _Fp> class __base;
+template<class _Fp> class _LIBCPP_TEMPLATE_VIS __base;
 
 template<class _Rp, class ..._ArgTypes>
 class __base<_Rp(_ArgTypes...)>