MAINTAINERS: Add entry for AF_XDP selftests files

MAINTAINERS: Add entry for AF_XDP selftests files

Lukas reported that after commit 710fd4f51ae7 ("libbpf: move xsk.{c,h}
into selftests/bpf") MAINTAINERS file needed an update.

In the meantime, Magnus removed AF_XDP samples in commit bd8bb5966ed5
("bpf, samples: Remove AF_XDP samples"), but selftests part still misses
its entry in MAINTAINERS.

Now that xdpxceiver became xskxceiver, tools/testing/selftests/bpf/*xsk*
will match all of the files related to AF_XDP testing (test_xsk.sh,
xskxceiver, xsk_prereqs.sh, xsk.{c,h}).

Reported-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220707111613.49031-3-maciej.fijalkowski@intel.com