mptcp: make userspace_pm_append_new_local_addr static

mptcp_userspace_pm_append_new_local_addr() has always exclusively been
used in pm_userspace.c since its introduction in
commit 4638de5a

 ("mptcp: handle local addrs announced by userspace PMs").

So make it static.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>