make: Do not scan contrib for discouraged functions

Makefile

@@ -490,7 +490,7 @@ check-tmpctx:
 	@if git grep -n 'tal_free[(]tmpctx)' | grep -Ev '^ccan/|/test/|^common/setup.c:|^common/utils.c:'; then echo "Don't free tmpctx!">&2; exit 1; fi
 
 check-discouraged-functions:
-	@if git grep -E "[^a-z_/](fgets|fputs|gets|scanf|sprintf)\(" -- "*.c" "*.h" ":(exclude)ccan/"; then exit 1; fi
+	@if git grep -E "[^a-z_/](fgets|fputs|gets|scanf|sprintf)\(" -- "*.c" "*.h" ":(exclude)ccan/" ":(exclude)contrib/"; then exit 1; fi
 
 # Don't access amount_msat and amount_sat members directly without a good reason
 # since it risks overflow.