net: filter: add test_bpf module under MAINTAINERS' networking section

Add lib/test_bpf.c entry to maintainers file under networking.
All changes were posted via netdev for review, so make sure
other people Cc it as well when they call get_maintainer.pl.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>