install meson for Documentation job

Why should we even do this?  Doesn't it run on ubuntu-latest which
has its own apt--get install for meson in the $distro specific
set-up before this part?

Author: gitster

ci/install-dependencies.sh

@@ -123,7 +123,7 @@ sparse)
 	;;
 Documentation)
 	sudo apt-get -q update
-	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make
+	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make meson
 
 	test -n "$ALREADY_HAVE_ASCIIDOCTOR" ||
 	sudo gem install --version 1.5.8 asciidoctor