drm_hwcomposer: CI: Make checkstyle a placeholder

The checkstyle job runs a quick script but currently makes use of a full
gitlab runner. Instead, add the placeholder tag to this job so that it
can be picked up and run in parallel with other light-weight jobs. This
should free up resources and avoid waits to run the checkstyle job.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index fbe2a85..3690573 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -56,3 +56,5 @@
   artifacts:
     when: on_failure
     untracked: true
+  tags:
+    - placeholder-job