icon-slicer: fix native build by depending on gdk-pixbuf only

| checking for GDK_PIXBUF... no
| configure: error: GTK+-2.0 is required to compile icon-slicer
<...>
ERROR: Task 2643 (virtual:native:/home/andreas/data/oe-core/sources/meta-openembedded/meta-oe/recipes-devtools/icon-slicer/icon-slicer_0.3.bb, do_configure) failed with exit code '1'

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Eric Bénard <eric@eukrea.com>