Codechange: Use override specifer for overriding member declarations

This is a C++11 feature that allows the compiler to check that a virtual
member declaration overrides a base-class member with the same signature.

Also src/blitter/32bpp_anim_sse4.hpp +38 is no longer erroneously marked
as virtual despite being a template.

src/newgrf_config.cpp

@@ -633,7 +633,7 @@ public:
 	{
 	}
 
-	/* virtual */ bool AddFile(const char *filename, size_t basepath_length, const char *tar_filename);
+	bool AddFile(const char *filename, size_t basepath_length, const char *tar_filename) override;
 
 	/** Do the scan for GRFs. */
 	static uint DoScan()