Adorn __call_once_proxy with inline and _LIBCPP_INLINE_VISIBILITY

As per discussion with mclow and EricWF on irc, this is small and
simple enough to deserve being inlined.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291497 91177308-0d34-0410-b5e6-96231b3b80d8

include/mutex

@@ -559,6 +559,7 @@ public:
 #endif
 
 template <class _Fp>
+inline _LIBCPP_INLINE_VISIBILITY
 void
 __call_once_proxy(void* __vp)
 {