avahi_0.6.25.bb: Fix build when DISTRO_FEATURES lacks ipv6.

avahi_0.6.25.bb: Fix build when DISTRO_FEATURES lacks ipv6.

socket.c: In function 'avahi_mdns_mcast_join_ipv6':
socket.c:159:38: error: 'IPV6_DROP_MEMBERSHIP' undeclared (first use in this function)
socket.c:159:38: note: each undeclared identifier is reported only once for each function it appears in
socket.c:161:45: error: 'IPV6_ADD_MEMBERSHIP' undeclared (first use in this function)
socket.c: In function 'ipv6_pktinfo':
socket.c:273:9: warning: unused variable 'yes'
socket.c:272:29: warning: unused parameter 'fd'
socket.c: In function 'avahi_open_socket_ipv6':
socket.c:387:38: error: 'IPV6_MULTICAST_HOPS' undeclared (first use in this function)
socket.c:393:38: error: 'IPV6_UNICAST_HOPS' undeclared (first use in this function)
socket.c:399:38: error: 'IPV6_V6ONLY' undeclared (first use in this function)
socket.c:405:38: error: 'IPV6_MULTICAST_LOOP' undeclared (first use in this function)
cache.c: In function 'avahi_cache_start_poof':
cache.c:494:5: warning: cast discards qualifiers from pointer target type
socket.c: In function 'avahi_send_dns_packet_ipv4':
socket.c:524:16: warning: cast increases required alignment of target type
socket.c: In function 'avahi_send_dns_packet_ipv6':
socket.c:575:23: error: invalid application of 'sizeof' to incomplete type 'struct in6_pktinfo'
socket.c:603:30: error: invalid application of 'sizeof' to incomplete type 'struct in6_pktinfo'
socket.c:608:27: error: 'IPV6_PKTINFO' undeclared (first use in this function)
socket.c:613:17: error: dereferencing pointer to incomplete type
socket.c:616:25: error: dereferencing pointer to incomplete type
socket.c: In function 'avahi_recv_dns_packet_ipv4':
socket.c:716:48: warning: cast increases required alignment of target type
socket.c:716:25: warning: dereferencing type-punned pointer will break strict-aliasing rules
socket.c:722:44: warning: cast increases required alignment of target type
socket.c: In function 'avahi_recv_dns_packet_ipv6':
socket.c:855:22: error: 'IPV6_HOPLIMIT' undeclared (first use in this function)
socket.c:858:48: warning: cast increases required alignment of target type
socket.c:858:25: warning: dereferencing type-punned pointer will break strict-aliasing rules
socket.c:864:22: error: 'IPV6_PKTINFO' undeclared (first use in this function)
socket.c:868:39: error: dereferencing pointer to incomplete type
socket.c:871:59: error: dereferencing pointer to incomplete type
socket.c: In function 'avahi_open_unicast_socket_ipv6':
socket.c:946:38: error: 'IPV6_V6ONLY' undeclared (first use in this function)
make[2]: *** [libavahi_core_la-socket.lo] Error 1

Signed-off-by: Graham Gower <graham.gower@gmail.com>
Acked-by: Khem Raj <raj.khem@gmail.com>