[gtk] ci: Use the appropriate argument for errors

[Emmanuele Bassi <ebassi src gnome org>]

commit cfeab26de0fa8279610badfe3f23534bbac193e9
Author: Emmanuele Bassi <ebassi gmail com>
Date:   Sun Mar 25 00:00:02 2018 +0000

    ci: Use the appropriate argument for errors
    
    The option to enable fatal warnings in Meson is `--werror`.

 .gitlab-ci/test-docker.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci/test-docker.sh b/.gitlab-ci/test-docker.sh
index 65cf246b62..fda2074f6f 100755
--- a/.gitlab-ci/test-docker.sh
+++ b/.gitlab-ci/test-docker.sh
@@ -9,7 +9,7 @@ meson \
         -Dwayland-backend=true \
         -Dbroadway-backend=true \
         -Dvulkan=yes \
-        -Dwerror=true \
+        --werror \
         _build $srcdir
 
 cd _build