openbox: add libcroco, librsvg and gdk-pixbuf to DEPENDS

fixes:

WARNING: QA Issue: openbox-core rdepends on libcroco, but it isn't a build dependency? [build-deps]
WARNING: QA Issue: openbox-core rdepends on librsvg, but it isn't a build dependency? [build-deps]
WARNING: QA Issue: openbox-core rdepends on gdk-pixbuf, but it isn't a build dependency? [build-deps]

why no PACKAGECONFIG?

* libcroco and gdk-pixbuf are not configurable
* librsvg is required to display svg icons which is a basic requirement

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>