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>