net: avoid including xdp.h in filter.h

If is sufficient with a forward declaration of struct xdp_rxq_info in
linux/filter.h, which avoids including net/xdp.h.  This was originally
suggested by John Fastabend during the review phase, but wasn't
included in the final patchset revision.  Thus, this followup.

Suggested-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>