package/gdk-pixbuf: add gobject-introspection support

package/gdk-pixbuf: add gobject-introspection support

This will fix the following build failure when enabling introspection on
libgtk2:

Couldn't find include 'GdkPixbuf-2.0.gir' (search path: '['/tmp/instance-0/output-1/host/bin/../mipsel-buildroot-linux-gnu/sysroot/usr/bin/../share/gir-1.0', '../gdk', '/tmp/instance-0/output-1/host/share', 'gir-1.0', '/tmp/instance-0/output-1/host/share/gir-1.0', '/usr/share/gir-1.0']')

Fixes:
 - http://autobuild.buildroot.org/results//86c6f55e0bd1a0fe3b70c9e97193aaad94d72a7f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>