[CI] Also test with clang.

There is a strange failure with pacxx.  Lets see whether we can reproduce it
with plain clang.

.gitlab-ci.yml

@@ -20,6 +20,11 @@ git--gcc:
   script:
   - bin/duneci-perftool-small-test
 
+git--clang:
+  image: docker.io/hpc2se/perftool-base:git
+  script:
+  - bin/duneci-perftool-small-test --opts=/duneci/opts.clang
+
 git--pacxx:
   image: docker.io/hpc2se/pacxx-perftools-base:git
   script: