package/gdk-pixbuf: disable introspection

package/gdk-pixbuf: disable introspection

Build will fail if host-gobject-introspection is built before
host-gdk-pixbuf:

Writing a loaders.cache file to use when running examples before installing gdk-pixbuf.
Couldn't find include 'GModule-2.0.gir' (search path: '['/home/fabrice/buildroot/output/host/share', '/usr/share/gnome/gir-1.0', '/usr/local/share/gir-1.0', '/usr/share/gir-1.0', '/home/fabrice/buildroot/output/host/share/gir-1.0', '/usr/share/gir-1.0']')

So disable introspection on target and host variant

Fixes:
 - No autobuilder failures yet

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>