netfilter: move nf_bridge_frag_data struct definition to a more appropriate header.

There is a struct definition function in nf_conntrack_bridge.h which is
not specific to conntrack and is used elswhere in netfilter.  Move it
into netfilter_bridge.h.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>