win32u: Make send_message_timeout() hidden.

dlls/win32u/win32u_private.h

@@ -314,7 +314,7 @@ extern LRESULT send_internal_message_timeout( DWORD dest_pid, DWORD dest_tid, UI
 extern LRESULT send_message( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam ) DECLSPEC_HIDDEN;
 extern BOOL send_notify_message( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam, BOOL ansi ) DECLSPEC_HIDDEN;
 extern LRESULT send_message_timeout( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam,
-                                     UINT flags, UINT timeout, BOOL ansi );
+                                     UINT flags, UINT timeout, BOOL ansi ) DECLSPEC_HIDDEN;
 
 /* rawinput.c */
 extern BOOL process_rawinput_message( MSG *msg, UINT hw_id, const struct hardware_msg_data *msg_data ) DECLSPEC_HIDDEN;