uwp: provide win32_dwm_last_vblank_time()

wgl_ctx.c and w_vk_ctx.c report the last present time through
win32_dwm_last_vblank_time(), which lives in win32_common.c, and that
file is not part of the UWP build. Add it to the UWP win32_* set: DWM
composition timing is not reachable from an app container, so it
reports no timestamp and the presenter paces on its own clock.

uwp/uwp_main.cpp

@@ -817,6 +817,13 @@ extern "C" {
       return App::GetInstance()->IsWindowFocused();
    }
 
+   /* DwmGetCompositionTimingInfo is not available to app containers,
+    * so the presenter paces on its own clock. */
+   retro_time_t win32_dwm_last_vblank_time(void)
+   {
+      return 0;
+   }
+
    bool win32_set_video_mode(void *data, unsigned width, unsigned height, bool fullscreen)
    {
       if (App::GetInstance()->IsInitialized())