AutoMoc: Restore support for re-running after project file changes

For Qt >= 5.15.0 and Ninja generators AutoMoc creates a depfile to let
Ninja decide when to run AutoMoc.  This was introduced by commit aebfbcaa
(AutoGen: Use depfiles for the XXX_autogen ninja targets, 2020-01-14,
v3.17.0-rc1~58^2).

However, AutoMoc was not triggered after adding a new moc-able file to
the project. This patch adds the project file (and potentially included
files) to the dependencies in the depfile.

Now, a re-run of AutoMoc is triggered if the project file changes.

Fixes: #21127