net: mdns: Fix compile error when using clang

No issues with gcc but clang gives this error for
the *v4 variable few lines below.

.../lib/dns/mdns_responder.c:712:2: error: expected expression
        struct net_context *v4;

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

subsys/net/lib/dns/mdns_responder.c

@@ -706,6 +706,9 @@ static int init_listener(void)
 		}
 	}
 ipv6_out:
+	; /* Added ";" to avoid clang compile error because of
+	   * the "struct net_context *v4" after it.
+	   */
 #endif /* CONFIG_NET_IPV6 */
 
 #if defined(CONFIG_NET_IPV4)