Replace StringToInt with the std version.

Again, RageUtil isn't the right home.

src/ScreenGameplay.cpp

@@ -2801,7 +2801,7 @@ void ScreenGameplay::SaveReplay()
 					continue;
 
 				ASSERT( matches.size() == 1 );
-				iIndex = StringToInt( matches[0] )+1;
+				iIndex = std::stoi( matches[0] )+1;
 				break;
 			}