librsvg: needs host-gdk-pixbuf

It's required for the gdk-pixbuf-query-loaders tool, to build the svg
gdk-pixbuf module. Fixes:
http://autobuild.buildroot.net/results/603/6031dbb937dadd04b6b9d1d77fc6106728502152/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>