Build-depend on gdk-pixbuf instead of deprecated GTK 2
This library actually needs gdk-pixbuf, and its only uses of GTK 2 were to pull in gdk-pixbuf as an indirect dependency, and to find the location into which to install a gdk-pixbuf loader.
Closes: #967585
I've verified using diffoscope that this change has no effect on the resulting binary packages. It significantly reduces the number of build-dependencies, so it would be worth applying even if GTK 2 wasn't obsolete and unmaintained.