win32: Fix build

gdkwindown-win32.c included windows.h directly rather than via gdkwin32.h
which broke the build for me at least. Instead rely on it being included in
gdkwin32.h and things work right.

gdk/win32/gdkwindow-win32.c

@@ -27,7 +27,6 @@
 
 #include "config.h"
 #include <stdlib.h>
-#include <windows.h>
 
 #include "gdk.h"
 #include "gdkwindowimpl.h"