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

MAINTAINERS

@@ -21507,6 +21507,7 @@ K:	(?:\b|_)xdp(?:\b|_)
 XDP SOCKETS (AF_XDP)
 M:	Björn Töpel <bjorn@kernel.org>
 M:	Magnus Karlsson <magnus.karlsson@intel.com>
+M:	Maciej Fijalkowski <maciej.fijalkowski@intel.com>
 R:	Jonathan Lemon <jonathan.lemon@gmail.com>
 L:	netdev@vger.kernel.org
 L:	bpf@vger.kernel.org