LibGUI: Include limits in Window

We use INT_MAX in a few places, but didn't explicitly load limits.h,
expecting that other system headers would pull it in.

Userland/Libraries/LibGUI/Window.cpp

@@ -29,6 +29,7 @@
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/Palette.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>