[gtk] ci: build with -Werror

[Benjamin Otte <otte src gnome org>]

commit b83cd9e818d0b8c71667f610d61431e74d6e429a
Author: Benjamin Otte <otte redhat com>
Date:   Sat Mar 24 22:27:28 2018 +0100

    ci: build with -Werror

 .gitlab-ci/test-docker.sh | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/.gitlab-ci/test-docker.sh b/.gitlab-ci/test-docker.sh
index 48b22d25ca..65cf246b62 100755
--- a/.gitlab-ci/test-docker.sh
+++ b/.gitlab-ci/test-docker.sh
@@ -9,6 +9,7 @@ meson \
         -Dwayland-backend=true \
         -Dbroadway-backend=true \
         -Dvulkan=yes \
+        -Dwerror=true \
         _build $srcdir
 
 cd _build