MAINTAINERS: Add entry for AF_XDP selftests files

Lukas reported that after commit f366006 ("libbpf: move xsk.{c,h}
into selftests/bpf") MAINTAINERS file needed an update.

In the meantime, Magnus removed AF_XDP samples in commit cfb5a2d
("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 <[email protected]>
Signed-off-by: Maciej Fijalkowski <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

MAINTAINERS

@@ -21749,6 +21749,7 @@ F:	include/uapi/linux/if_xdp.h
 F:	include/uapi/linux/xdp_diag.h
 F:	include/net/netns/xdp.h
 F:	net/xdp/
+F:	tools/testing/selftests/bpf/*xsk*
 
 XEN BLOCK SUBSYSTEM
 M:	Roger Pau Monné <[email protected]>