mptcp: drop duplicate mptcp_setsockopt() declaration

commit 78962489 ("mptcp: add skeleton to sync msk socket
options to subflows") introduced a duplicate declaration of
mptcp_setsockopt(), just drop it.

Reported-by: Florian Westphal <fw@strlen.de>
Fixes: 78962489 ("mptcp: add skeleton to sync msk socket options to subflows")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>