github-ci: test make after make clean

On the CentOS 7 build, test a make after a make clean. Should
catch the case where bundled generated headers files get deleted
when cbindgen is not available to rebuild them.

.github/workflows/builds.yml

@@ -152,6 +152,8 @@ jobs:
       - run: make install
       - run: make install-conf
       - run: make distcheck
+      - run: make clean
+      - run: make -j2
 
   centos-6:
     name: CentOS 6