added multimedia repos to ci

.gitlab-ci.yml

@@ -2,6 +2,9 @@ buildtest:
   image: debian:stretch
   stage: test
   script:
+    - 'echo "deb http://www.deb-multimedia.org stretch main non-free" >> /etc/apt/sources.list'
+    - wget https://www.deb-multimedia.org/pool/main/d/deb-multimedia-keyring/deb-multimedia-keyring_2016.8.1_all.deb
+    - dpkg -i deb-multimedia-keyring_2016.8.1_all.deb
     - apt update
     - uname -a
     - apt install -y locales-all git build-essential  make libavformat-dev libavcodec-dev libavfilter-dev libavutil-dev libxml2-dev libcurl4-openssl-dev