CommonHostInterface: Add onscreen indicator for rewind

src/core/system.h

@@ -237,6 +237,7 @@ void CalculateRewindMemoryUsage(u32 num_saves, u64* ram_usage, u64* vram_usage);
 void ClearMemorySaveStates();
 void UpdateMemorySaveStateSettings();
 bool LoadRewindState(u32 skip_saves = 0, bool consume_state = true);
+bool IsRewinding();
 void SetRewinding(bool enabled);
 void SetRunaheadReplayFlag();