The generated "changelog" was not removed
before re-generating, causing the changelog
to be added twice on repeated runs of "make rpm"
(when bind-mounting the local source).

As a result, rpms failed to build, because
the resulting file had entries in non-chronological
order.

This change removes the generated file
before re-generating, and adds the file
to .gitignore, to prevent it from
accidentally being added to source control.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>