[mutter] gitlab-ci.yml: Treat warnings as errors

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

commit 6c91683380e138472d11900be352f24ed9105c40
Author: Jonas Ådahl <jadahl gmail com>
Date:   Sun Jan 20 19:27:39 2019 +0100

    gitlab-ci.yml: Treat warnings as errors
    
    This'll allow us to catch them before they land.

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 6237626cf..9c7fbe42c 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -7,7 +7,7 @@ stages:
 build-mutter:
   stage: build
   script:
-    - meson . build -Degl_device=true -Dwayland_eglstream=true
+    - meson . build -Degl_device=true -Dwayland_eglstream=true --werror
     - ninja -C build
     - ninja -C build install