CI: Enable warnings as errors

Enable -Werror by setting -Dwerror=true on the Meson command line.
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>

test/gitlab-ci.sh

 #!/bin/sh
 set -xe
 
-meson setup build/
-meson configure -Dprefix=/usr -Ddri2=true -Ddocumentation=true build/
+meson setup -Dprefix=/usr -Ddri2=true -Ddocumentation=true -Dwerror=true build/
 ninja -C build/ install test
 cp -av build/doc/html public/