libretro: EventCycles must match definition in gpu.cpp (signed)

... and as a rule of thumb, always favor signed int unless there is specific reason to use unsigned (normally limited to situations where full 4GB unsigned range is required during comparison operation)

libretro.cpp

@@ -108,7 +108,7 @@ int memfd;
 #endif
 #endif
 
-uint32 EventCycles = 128;
+int32 EventCycles = 128;
 
 // CPU overclock factor (or 0 if disabled)
 int32_t psx_overclock_factor = 0;