package/libgtk3: disable introspection

package/libgtk3: disable introspection

Build will fail if gobject-introspection is built before libgtk3:

  GISCAN   Gdk-3.0.gir
Couldn't find include 'GdkPixbuf-2.0.gir' (search path: '['/home/fabrice/buildroot/output/host/bin/../aarch64-buildroot-linux-gnu/sysroot/usr/bin/../share/gir-1.0', '../gdk', '/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']')
make[5]: *** [/home/fabrice/buildroot/output/host/bin/../aarch64-buildroot-linux-gnu/sysroot/usr/lib/../share/gobject-introspection-1.0/Makefile.introspection:156: Gdk-3.0.gir] Error 1

So disable introspection on host and target 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>