MAINTAINERS: add reviewer to maintainers entry

Jonathan Lemon has volunteered as an official AF_XDP reviewer. Thank
you, Jonathan!
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

MAINTAINERS

@@ -17265,6 +17265,7 @@ N:	xdp
 XDP SOCKETS (AF_XDP)
 M:	Björn Töpel <bjorn.topel@intel.com>
 M:	Magnus Karlsson <magnus.karlsson@intel.com>
+R:	Jonathan Lemon <jonathan.lemon@gmail.com>
 L:	netdev@vger.kernel.org
 L:	bpf@vger.kernel.org
 S:	Maintained