LibGfx: IWYU for FourCC.h

Makes clangd-induced red squigglies disappear when editing this file.

No behavior change.

Userland/Libraries/LibGfx/FourCC.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/Types.h>
+
 namespace Gfx {
 
 struct [[gnu::packed]] FourCC {