Skip functional tests for .gitignore update

The file itself has no functional impact.

Change-Id: Iad4e6fc988dcf98ba42c94d8502fc6a60e49e520
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

.zuul.yaml

@@ -6,6 +6,7 @@
     vars:
       tox_envlist: functional
     irrelevant-files:
+      - ^\.gitignore$
       - ^\.gitreview$
       - ^.*\.rst$
       - ^doc/.*$