nfp: Explicitly include linux/bug.h

Today's -next build encountered an error due to a missing definition of
WARN_ON(), caused by some header reorganization removing an implicit
inclusion of linux/bug.h.  Fix this with an explicit inclusion.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>