CI: enforce lack of carriage return / tab

.gitlab-ci.yml

 stages:
+    - style
     - build
     - test
 
+style-check:
+    image: registry.videolan.org:5000/dav1d-debian-unstable:20181114201132
+    stage: style
+    tags:
+        - debian
+        - amd64
+    script:
+        - git grep -n -e $'\t' --or -e $'\r' -- . ':(exclude)*/compat/*' && exit 1
+        - /bin/true
+
 build-debian:
     image: registry.videolan.org:5000/dav1d-debian-unstable:20181114201132
     stage: build