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>

.gitlab-ci.d/buildtest.yml

@@ -353,6 +353,8 @@ build-tcg-disabled:
             124 132 139 142 144 145 151 152 155 157 165 194 196 200 202
             208 209 216 218 227 234 246 247 248 250 254 255 257 258
             260 261 262 263 264 270 272 273 277 279 image-fleecing
+    - cd ../..
+    - make distclean
 
 build-user:
   extends: .native_build_job_template

contrib/plugins/Makefile

@@ -77,7 +77,7 @@ lib%$(SO_SUFFIX): %.o
 endif
 
 
-clean:
+clean distclean:
 	rm -f *.o *$(SO_SUFFIX) *.d
 	rm -Rf .libs