netfilter: add missing IS_ENABLED(CONFIG_BRIDGE_NETFILTER) checks to header-file.

br_netfilter.h defines inline functions that use an enum constant and
struct member that are only defined if CONFIG_BRIDGE_NETFILTER is
enabled.  Added preprocessor checks to ensure br_netfilter.h will
compile if CONFIG_BRIDGE_NETFILTER is disabled.

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