gitlab-ci: flip the error on any warning

Flip the meson werror=true switch. It will help us catch extra issues
before they creep in.

Closes: mesa/waffle#73
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
4 jobs for warning-cleanups in 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Container
passed container #11772294

00:00:20

 
  Build
passed cmake #11772296

00:00:24

passed cmake-mingw #11772297

00:00:26

passed meson #11772295

00:00:27