MAINTAINERS: Add maintainer to AF_XDP

Maciej Fijalkowski has gracefully accepted to become the third
maintainer for the AF_XDP code. Thank you Maciej!

Signed-off-by: Magnus Karlsson <magnus.karlsson@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20220523083254.32285-1-magnus.karlsson@gmail.com