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/game/game_info.hpp

@@ -40,7 +40,7 @@ public:
 	 */
 	const char *GetAPIVersion() const { return this->api_version; }
 
-	/* virtual */ bool IsDeveloperOnly() const { return this->is_developer_only; }
+	bool IsDeveloperOnly() const override { return this->is_developer_only; }
 
 private:
 	int min_loadable_version; ///< The Game can load savegame data if the version is equal or greater than this.