Add perf stat logging through ini setting

For better tracking of performance regressions on incoming changes, this
change adds a way to dump frametime to file by changing an ini config
option. This is intentionally hidden as its only useful to a small
number of individuals, and not really applicable to the general
userbase.

src/core/settings.h

@@ -187,6 +187,7 @@ struct Values {
     std::array<int, Service::CAM::NumCameras> camera_flip;
 
     // Debugging
+    bool record_frame_times;
     bool use_gdbstub;
     u16 gdbstub_port;
     std::string log_filter;