ci: Add libx264 lib to doc jobs

Author: edalm

utils/tests/gitlab-ci-doc.yml

@@ -7,7 +7,7 @@
     - pacman-key --init
     - pacman -Syu --noconfirm
       python python-sphinx make libxml2-legacy
-      graphviz imagemagick
+      graphviz imagemagick libx264
       texlive-basic texlive-bin texlive-binextra texlive-latexextra texlive-fontutils texlive-fontsrecommended texlive-mathscience
     # dia is archived
     - pacman --noconfirm -U