gdk-win32: fix unused variable warnings

gdk-win32: fix unused variable warnings

../gdk/win32/gdksurface-win32.c: In function 'gdk_win32_get_window_size_and_position_from_client_rect':
../gdk/win32/gdksurface-win32.c:3753:20: warning: unused variable 'impl' [-Wunused-variable]
 3753 |   GdkWin32Surface *impl = GDK_WIN32_SURFACE (window);
      |                    ^~~~
../gdk/win32/gdksurface-win32.c: In function 'gdk_win32_surface_get_queued_window_rect':
../gdk/win32/gdksurface-win32.c:5129:20: warning: unused variable 'impl' [-Wunused-variable]
 5129 |   GdkWin32Surface *impl = GDK_WIN32_SURFACE (surface);

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>