mptcp: move mptcp_pm_gen.h's include

Nothing from protocol.h depends on mptcp_pm_gen.h, only code from
pm_netlink.c and pm_userspace.c depends on it.

So this include can be moved where it is needed to avoid a "unused
includes" warning.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20240514011335.176158-8-martineau@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>