Skip functional tests for .gitignore update

... because the file itself has no functional impact.

Change-Id: I6e5118c24663d8e1921786e9edbbc7b64eadedda
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

.zuul.yaml

@@ -10,6 +10,7 @@
       devstack_plugins:
         heat: https://opendev.org/openstack/heat
     irrelevant-files:
+      - ^\.gitignore$
       - ^\.gitreview$
       - ^\.pre-commit-config\.yaml$
       - ^(test-|)requirements.txt$