UICommon/NetPlayIndex: Take NetPlaySession by const reference for Add()

This isn't std::moved wholesale into a member variable or further
std::moved into another function, so it's better to take it by const
reference here to avoid unnecessary reallocations of contained
std::string instances.

Source/Core/UICommon/NetPlayIndex.h

@@ -44,7 +44,7 @@ public:
 
   static std::vector<std::pair<std::string, std::string>> GetRegions();
 
-  bool Add(NetPlaySession session);
+  bool Add(const NetPlaySession& session);
   void Remove();
 
   bool HasActiveSession() const;