Fix shared_mutex dll import errors on Windows

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302394 91177308-0d34-0410-b5e6-96231b3b80d8

include/shared_mutex

@@ -177,7 +177,7 @@ class _LIBCPP_TYPE_VIS _LIBCPP_AVAILABILITY_SHARED_MUTEX shared_mutex
 {
     __shared_mutex_base __base;
 public:
-    shared_mutex() : __base() {}
+    _LIBCPP_INLINE_VISIBILITY shared_mutex() : __base() {}
     _LIBCPP_INLINE_VISIBILITY ~shared_mutex() = default;
 
     shared_mutex(const shared_mutex&) = delete;