netfilter: xt_socket: make module available for INPUT chain

This should make it possible to test for the existence of local
sockets in the INPUT path.

References: http://marc.info/?l=netfilter-devel&m=125380481517129&w=2



Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>