Add librsvg2-bin/-tools to bindep.txt

Trying to build docs locally gave the following error:

Warning, treated as error:
RSVG converter command 'rsvg-convert' cannot be run. Check the rsvg_converter_bin setting

When I install librsvg2-bin on my (ubuntu) system, the build succeeds.

This commit creates a bindep.txt with librsvg2-bin/-tools to resolve
this.

Change-Id: Ide80ef527c94e86ebb54aee6d319fd41931e205b

bindep.txt

@@ -0,0 +1,6 @@
+# This is a cross-platform list tracking distribution packages needed for install and tests;
+# see https://docs.openstack.org/infra/bindep/ for additional information.
+
+# libsrvg2 is needed for sphinxcontrib-svg2pdfconverter in docs builds.
+librsvg2-tools [doc platform:rpm]
+librsvg2-bin [doc platform:dpkg]