bpfilter: fix build dependency

BPFILTER could have been enabled without INET causing this build error:
ERROR: "bpfilter_process_sockopt" [net/bpfilter/bpfilter.ko] undefined!

Fixes: d2ba09c1

 ("net: add skeleton of bpfilter kernel module")
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>