Add missing font for PDF generation

The openstack-tox-docs job started failing with the following error:

    ! LaTeX Error: File `tgtermes.sty' not found.

This change adds the required font package to the bindep doc profile.

Change-Id: I9acf8fd03150b9cc03a011364f0c45929b9a9f87

bindep.txt

 libpcre3-dev [platform:dpkg test]
 pcre-devel [platform:rpm test]
+
+# PDF Docs package dependencies
+tex-gyre [platform:dpkg doc]