add a lookup enum never to escape HSH_Lookup

This is caught by

	assert(lr == HSH_HIT || lr == HSH_GRACE);

in cnt_lookup()

bin/varnishd/hash/hash_slinger.h

@@ -50,6 +50,7 @@ struct hash_slinger {
 };
 
 enum lookup_e {
+	HSH_CONTINUE,
 	HSH_MISS,
 	HSH_BUSY,
 	HSH_HIT,