Define and initialise PsxClockSpeed

- Fixes crash on entering bios

libpcsxcore/psxcommon.c

@@ -34,6 +34,9 @@ FILE *emuLog = NULL;
 u32 rewind_counter=0;
 u8 vblank_count_hideafter=0;
 
+// Used for overclocking
+u32 PsxClockSpeed = 33868800;
+
 int EmuInit() {
 	return psxInit();
 }

libpcsxcore/psxcommon.h

@@ -187,7 +187,7 @@ extern u8 vblank_count_hideafter;
 // Make the timing events trigger faster as we are currently assuming everything
 // takes one cycle, which is not the case on real hardware.
 // FIXME: Count the proper cycle and get rid of this
-u32 PsxClockSpeed;
+extern u32 PsxClockSpeed;
 #define BIAS	2
 #define PSXCLK	PsxClockSpeed	/* 33.8688 MHz */