Boot: Fix handling of M3U file paths containing backslashes

Previously, if a user on Windows launched Dolphin from the command line
and specified a path to an M3U file and included backslashes in this path,
Dolphin would fail to resolve relative paths in the M3U file.

Source/Core/Core/Boot/Boot.cpp

@@ -216,7 +216,13 @@ std::unique_ptr<BootParameters> BootParameters::GenerateFromFile(std::vector<std
 
   std::string folder_path;
   std::string extension;
+#ifdef HAS_STD_FILESYSTEM
+  const std::filesystem::path converted_path = StringToPath(paths.front());
+  folder_path = PathToString(converted_path.parent_path());
+  extension = PathToString(converted_path.extension());
+#else
   SplitPath(paths.front(), &folder_path, nullptr, &extension);
+#endif
   Common::ToLower(&extension);
 
   if (extension == ".m3u" || extension == ".m3u8")