mptcp: include inet_common in mib.h

So this file is now self-contained: it can be compiled alone with
analytic tools.

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/20240514011335.176158-9-martineau@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>