Bug#966394: libgtk-3-bin: Add dependency on librsvg2-common

[Colomban Wendling]

Package: libgtk-3-bin
Version: 3.24.24-4+deb11u2
Followup-For: Bug #966394

Hello,

I just got bit by this upgrading a custom package from Buster to
Bullseye, where the old provider for gtk-encode-symbolic-svg
(gtk-3-examples) did somehow bring the required loader, while the new
libgtk-3-bin does not.

I agree with Dan that given this totally breaks one of the tools this
package provides it should be fixed with a plain Depends.

Anyway, for the moment I fixed my package by simply adding the
dependency manually, just like I depend on libsrvg2-bin for
imagemagick's `convert` SVG support, but that one makes more sense as
it's merely one of the many formats the `convert` tool supports.

- Colomban