windows: make uv_rwlock_init initialize libuv

Without this patch, the fallback implementation would be used if
uv_rwlock_init were to be called before a loop was created or
uv_default_loop() was called.

src/win/thread.c

@@ -140,6 +140,8 @@ void uv_mutex_unlock(uv_mutex_t* mutex) {
 
 
 int uv_rwlock_init(uv_rwlock_t* rwlock) {
+  uv__once_init();
+
   if (HAVE_SRWLOCK_API())
     return uv__rwlock_srwlock_init(rwlock);
   else