fix format warnings

Enable format security, and fix the warning caused by printing
with string for format.
diff --git a/ip/ipaddress.c b/ip/ipaddress.c
index ce5e762..76f4782 100644
--- a/ip/ipaddress.c
+++ b/ip/ipaddress.c
@@ -357,7 +357,7 @@
 		fprintf(fp, "    TX errors: aborted fifo    window  heartbeat");
                 if (carrier_changes)
 			fprintf(fp, " transns");
-		fprintf(fp, _SL_);
+		fprintf(fp, "%s", _SL_);
 		fprintf(fp, "               %-7"PRIu64"  %-7"PRIu64" %-7"PRIu64" %-8"PRIu64"",
 			(uint64_t)s->tx_aborted_errors,
 			(uint64_t)s->tx_fifo_errors,
@@ -406,7 +406,7 @@
 		fprintf(fp, "    TX errors: aborted fifo    window  heartbeat");
                 if (carrier_changes)
 			fprintf(fp, " transns");
-		fprintf(fp, _SL_);
+		fprintf(fp, "%s", _SL_);
 		fprintf(fp, "               %-7u  %-7u %-7u %-8u",
 			s->tx_aborted_errors,
 			s->tx_fifo_errors,