libiptcdata fails to cross build from source for three distinct reasons.

1. python3-all-dev is not satisfiable. For cross building, this should
   become python3-all-dev:native plus libpython3-all-dev.
2. python3-all-dev:native and libpython3-all-dev are not coinstallable.
   Reported as #1121810.
3. The gtk-doc scanner fails. Since the documentation has been separated
   to an Arch:all -doc package, gtk-doc can be skipped in arch-only
   builds.

I'm attaching a patch resolving problems 1 and 3. While this doesn't
make libiptcdata cross buildable yet, it is an incremental step towards
that goal. Please consider applying the patch. Do close the bug even if
libiptcdata does not become cross buildable.

Helmut