Ignore non-existent .ui file dependencies

Ignore non-existent .ui file dependencies

Origin: upstream, https://code.qt.io/cgit/qt/qtbase.git/commit/?id=90779d96adfa51a3
Last-Update: 2019-11-05

Commit 80dea664 broke .ui files with global includes that are not part
of the project, because we blindly added every file path that falls
out of 'uic -d' as dependency.

Introduce the extra compiler CONFIG flag dep_existing_only to bring
back the old behavior that ignores non-existent dependencies and set
it for uic.

Gbp-Pq: Name uic_global_includes.diff