Core: Remove some header inclusions in header files

Replaces them with forward declarations of used types, or removes them entirely if they aren't used at all. This also replaces certain Common headers with less inclusive ones (in terms of definitions they pull in).

Source/Core/Core/HW/Sram.cpp

@@ -2,6 +2,7 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
+#include "Common/FileUtil.h"
 #include "Core/ConfigManager.h"
 #include "Core/HW/Sram.h"