GameList: Convert file extensions to lowercase

This means that dol/elf files with upercase extensions (e.g. BOOT.DOL) are properly detected.

Source/Core/UICommon/GameFile.cpp

@@ -387,6 +387,7 @@ std::string GameFile::GetExtension() const
 {
   std::string extension;
   SplitPath(m_file_path, nullptr, nullptr, &extension);
+  Common::ToLower(&extension);
   return extension;
 }