contrib/plugins/Makefile: Add a 'distclean' target

contrib/plugins/Makefile: Add a 'distclean' target

Running "make distclean" in the build tree currently fails since this
tries to run the "distclean" target in the contrib/plugins/ folder, too,
but the Makefile there is missing this target. Thus add 'distclean' there
to fix this issue.

And to avoid regressions with "make distclean", add this command to one
of the build jobs, too.

Message-ID: <20240902154749.73876-1-thuth@redhat.com>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>