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>