detect/iponly: fix compile warning

When --enable-unittests w/o --enable-debug is used.

src/detect-engine-iponly.c

@@ -506,6 +506,7 @@ static void IPOnlyCIDRListPrint(IPOnlyCIDRItem *tmphead)
                    (tmphead->negated) ? "yes":"no",
                    inet_ntoa(*(struct in_addr*)&tmphead->ip[0]),
                    tmphead->signum);
+        (void)i;
         tmphead = tmphead->next;
     }
 }