kqueue: Add a no-op EV_ENABLE definition.

EV_ADD implies ENABLE, so this isn't needed, but a lot of applications
try to specify it anyway. So this extends source compatibility.

If in the future we ever implement EV_DISABLE, we can just add a
real definition for it then, and since ADD implies ENABLE existing
applications shouldn't break.

headers/compatibility/bsd/sys/event.h

@@ -86,6 +86,7 @@ struct kevent {
 /* actions */
 #define EV_ADD		0x0001		/* add event to kq (implies enable) */
 #define EV_DELETE	0x0002		/* delete event from kq */
+#define EV_ENABLE	(0)			/* (not needed) */
 
 /* flags */
 #define EV_ONESHOT	0x0010		/* only report one occurrence */