Skip litmus job for .pdkignore update

This was missed during the previous attempt to skip most of CI tests
for .pdkignore update, which does not affect functionality.

Change-Id: Ib0d6f010709122f556c5266f2225ca160ddfdaaf

zuul.d/litmus.yaml

@@ -22,6 +22,7 @@
       - ^LICENSE$
       - ^.gitreview$
       - ^.gitignore$
+      - ^.pdkignore$
       - ^setup.cfg$
       - ^setup.py$
       - ^tox.ini$