Documentation: Add minimum pahole version

A report was made in https://github.com/acmel/dwarves/issues/26 about
pahole not being listed in the process/changes.rst file as being needed
for building the kernel, address that.

Link: https://github.com/acmel/dwarves/issues/26
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/YZPQ6+u2wTHRfR+W@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/r/YZfzQ0DvHD5o26Bt@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>