kernel: fix xt_connmark.h

Commit a1cfe0dc (kernel: connmark set-dscpmark follow upstreamimg
attempt") broke the usage of xt_connmark.h in user-space (e.g.
strongswan), because the BIT() macro is unknown there.

Fixes: a1cfe0dc (kernel: connmark set-dscpmark follow upstreamimg attempt")
Signed-off-by: Martin Schiller <ms@dev.tdt.de>