Increase dependency on librsvg2-common from Suggests to Recommends

This is not a hard dependency, but should be installed in nearly all
cases. Increasingly many icons are provided in SVG format, so
applications will appear broken if the SVG pixbuf loader is not
installed. See #980396 for more information.

adwaita-icon-theme already Recommends librsvg2-common, but people who
routinely do not install recommended packages will get a better hint
about how much will be broken by its removal if GTK also recommends it.