Fix: GCC warning

net/mptcp/mptcp_ipv6.c: In function 'mptcp_v6v4_syn_recv_sock':
net/mptcp/mptcp_ipv6.c:235:2: warning: value computed is not used [-Wunused-value]

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>