[gnome-control-center] ci: Properly quote regex string

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 425feb6e5914ddd2ac2e3fe8be5c979d85664a7f
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Sun Nov 18 21:50:04 2018 -0200

    ci: Properly quote regex string

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 95541d75f..5dabe4b86 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -323,7 +323,7 @@ ppc64le:
   script:
     # Skip the build (if requested)
     - |
-      if [[ -z "${CI_COMMIT_MESSAGE##*[skip images]*}" ]]; then
+      if [[ -z "${CI_COMMIT_MESSAGE##*'[skip images]'*}" ]]; then
         echo "== Nothing to do =="
         exit 0
       fi