Fix transitive includes in GraphicsUtil.cpp

I ran Include What You Use on the file, and a BUNCH of transitive
includes showed up.

colourTransform is used in the file, so GraphicsUtil.h needs to be
included. libc floor() is used in the file, so math.h needs to be
included (I'm removing this next...). NULL is used, so stddef.h. And
stdlib.h is used because we use rand() directly instead of going through
fRandom(). Speaking of which, we use fRandom(), so Maths.h needs to be
included, too.

desktop_version/src/GraphicsUtil.cpp

@@ -1,4 +1,12 @@
+#include "GraphicsUtil.h"
+
+#include <math.h>
+#include <SDL.h>
+#include <stddef.h>
+#include <stdlib.h>
+
 #include "Graphics.h"
+#include "Maths.h"