Fix incompatible pointers of the generic PIT and PCjr's

So that the build compiles fine.

src/machine/m_pcjr.c

@@ -772,7 +772,7 @@ speed_changed(void *priv)
 }
 
 void
-pit_irq0_timer_pcjr(int new_out, int old_out)
+pit_irq0_timer_pcjr(int new_out, int old_out, UNUSED(void *priv))
 {
     if (new_out && !old_out) {
         picint(1);

src/machine/machine.c

@@ -156,7 +156,7 @@ machine_available(int m)
 }
 
 void
-pit_irq0_timer(int new_out, int old_out)
+pit_irq0_timer(int new_out, int old_out, UNUSED(void *priv))
 {
     if (new_out && !old_out)
         picint(1);