autotools: Include the manpage in the tarball.

Unlike most source files, manpages are not included in the
automake dist target by default, on the assumption they're
generated from some other source.

Since our manpage is actually written in man, mark it for
inclusion explicitly. Fixes `make distcheck`.
1 job for gitlab-ci in 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #10955
docker
autotools

00:00:28