Fix memory leak in config

Assigning a pointer to std::string appears to only perform a copy and
does not transfer ownership of the pointer, thus the allocated memory is
will never be freed. Use StringBuilder::GetStdString() to avoid the
memory leak.

src/openrct2/config/IniReader.cpp

@@ -354,7 +354,7 @@ private:
                 sb.Append(&c, 1);
             }
         }
-        return std::string(sb.GetString());
+        return sb.GetStdString();
     }
 
     std::string GetLine(size_t index)