MAINTAINERS: Add a maintainer for bpftool

I've been contributing and reviewing patches for bpftool for some time,
and I'm taking care of its external mirror. On Alexei, KP, and Daniel's
suggestion, I would like to step forwards and become a maintainer for
the tool. This patch adds a dedicated entry to MAINTAINERS.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: KP Singh <kpsingh@kernel.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220608121428.69708-1-quentin@isovalent.com