ip xfrm mon: Add objects list to the usage output

Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

ip/xfrm_monitor.c

@@ -36,7 +36,8 @@ static void usage(void) __attribute__((noreturn));
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: ip xfrm monitor [ all | LISTofXFRM-OBJECTS ]\n");
+	fprintf(stderr, "Usage: ip xfrm monitor [ all | OBJECTS | help ]\n");
+	fprintf(stderr, "OBJECTS := { acquire | expire | SA | aevent | policy | report }\n");
 	exit(-1);
 }