built_in: fix build if ARPHRD_IEEE802154_MONITOR is undefined

Some 3.x kernel versions seem not to ship yet the definition of
ARPHRD_IEEE802154_MONITOR. Add it into built_in.h to let the
tools build.

Reported-by: Pablo Henrique <phenrique@gmail.com>
Reported-by: Tobias Kalbitz <tobias.kalbitz@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>