config: Pass strings by const reference where applicable

In a few cases, the strings aren't ever directly modified, so they can
be passed by reference to eliminate a few trivial copies.

External/FEXCore/Source/Interface/Config/Config.cpp

@@ -83,7 +83,7 @@ namespace FEXCore::Config {
     return ConfigFile;
   }
 
-  std::string GetApplicationConfig(std::string &Filename, bool Global) {
+  std::string GetApplicationConfig(const std::string &Filename, bool Global) {
     std::string ConfigFile = GetConfigDirectory(Global);
     if (!Global &&
         !std::filesystem::exists(ConfigFile) &&