Docs: Add requirement of librsvg2-bin for doc generation

Require tool librsvg2-bin in docs for user guide generation.

librsvg2-bin shall be installed to support
sphinxcontrib-svg2pdfconverter.
Otherwise, user guide build will throw out warnings that RSVG converter
command 'rsvg-convert' cannot be run.

Change-Id: Ic988b2771c0c5317b130bb37696c1c2833c162f7
Signed-off-by: David Hu <david.hu@arm.com>
2 files changed