Add a compat API shim for ithread_remove_handler().

Approved by:	re (mux)

sys/kern/kern_intr.c

@@ -614,6 +614,7 @@ swi_remove(void *cookie)
 }
 
 /* ABI compatibility shim. */
+#undef ithread_remove_handler
 int	ithread_remove_handler(void *);
 
 int

sys/sys/interrupt.h

@@ -36,6 +36,7 @@
 #define	tty_intr_event	tty_ithd
 #define	clk_intr_event	clk_ithd
 #define	ithd		intr_event
+#define	ithread_remove_handler	intr_event_remove_handler
 
 struct intr_event;
 struct intr_thread;