Timers: Handle target = 0 case

Fixes Super Gals! Kotobuki Ran Special - Ikemen Get You Gals Party
hanging at boot.

src/core/timers.h

@@ -85,6 +85,7 @@ private:
   };
 
   void UpdateCountingEnabled(CounterState& cs);
+  void CheckForIRQ(u32 index, u32 old_counter);
   void UpdateIRQ(u32 index);
 
   void AddSysClkTicks(TickCount sysclk_ticks);