GameList: Convert file extensions to lowercase

[Pokechu22]

This means that dol/elf files with upercase extensions (e.g. BOOT.DOL) are properly detected. ISO files with uppercase extensions already worked. This affected Rick Dangerous Wii (though it is still broken in other ways).

Note that the same case conversion is performed by Boot.cpp:

dolphin/Source/Core/Core/Boot/Boot.cpp

Lines 205 to 208 in 07fd174

	std::string folder_path;
	std::string extension;
	SplitPath(paths.front(), &folder_path, nullptr, &extension);
	std::transform(extension.begin(), extension.end(), extension.begin(), ::tolower);