user32: Add a dummy _wassert implementation to avoid a Mingw build issue.

_wassert references __imp__MessageBoxW@16 which causes a duplicate
definition of MessageBoxW.

Signed-off-by: Alexandre Julliard <julliard@winehq.org>

dlls/user32/user_main.c

@@ -55,6 +55,10 @@ static DWORD exiting_thread_id;
 
 extern void WDML_NotifyThreadDetach(void);
 
+#ifdef __MINGW32__
+/* work around a Mingw build issue where _wassert causes a duplicate reference to MessageBoxW */
+void __cdecl _wassert( const WCHAR *msg, const WCHAR *file, unsigned line) { abort(); }
+#endif
 
 /***********************************************************************
  *           USER_Lock