gitlab-ci: Switch to clang-tools

clang-3.9 does not contain scan-build anymore.

.gitlab-ci.yml

@@ -32,7 +32,7 @@ build-debian-clang:
   image: debian:buster
   before_script:
     - apt-get -y update
-    - apt-get -y install build-essential clang-3.9
+    - apt-get -y install build-essential clang-tools
     - apt-get -y build-dep .
   <<: *tags
   stage: build