scangobj: Child properties are gone

If we are handling GTK >= 3.96.0, child properties
don't exist anymore, so don't try to introspect them.
3 jobs for gtk4-fixes in 12 minutes and 52 seconds
latest
Status Job ID Name Coverage
  Build
passed #333959
autotools-build

00:07:33

passed #333960
meson-build

00:07:07

 
  Test
passed #333961
autotools-test

00:05:19

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