Build-depend on libgdk-pixbuf-2.0-dev instead of libgtk2.0-dev

Simon McVittierequested to merge
wip/bug967580 into master

GTK 2 is obsolete, but this package was relying on libgtk2.0-dev to pull in gdk-pixbuf (which is not obsolete) as a dependency. Swap the build-dependency to the library that is actually used.

Closes: #967580

This also considerably reduces the number of packages pulled in as build-dependencies, so it would be a desirable change even if we were not trying to reduce the number of things depending on GTK 2.

I have confirmed using diffoscope that this change does not alter the contents of the binary packages.

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