[CI] Enable a post-install header test.

Author: hageboeck

.github/workflows/root-ci.yml

@@ -585,6 +585,9 @@ jobs:
         working-directory: /github/home/ROOT-CI/PostInstall
         run: ctest -j $(nproc)
 
+      - name: Check installed headers
+        run: bash test/PostInstall/check-headers.sh /github/home/ROOT-CI/install/include
+
   event_file:
     # For any event that is not a PR, the CI will always run. In PRs, the CI
     # can be skipped if the tag [skip-ci] or [skip ci] is written in the title.