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>

include/linux/filter.h

@@ -20,7 +20,6 @@
 #include <linux/set_memory.h>
 #include <linux/kallsyms.h>
 
-#include <net/xdp.h>
 #include <net/sch_generic.h>
 
 #include <uapi/linux/filter.h>
@@ -30,6 +29,7 @@ struct sk_buff;
 struct sock;
 struct seccomp_data;
 struct bpf_prog_aux;
+struct xdp_rxq_info;
 
 /* ArgX, context and stack frame pointer register positions. Note,
  * Arg1, Arg2, Arg3, etc are used as argument mappings of function