Consider a Qt project with a header file that does not contain the
Q_OBJECT macro. Adding the Q_OBJECT macro is supposed to trigger a run
of moc.

When using Qt >= 5.15 and the Ninja generator, re-running AutoMoc is
controlled by the contents of a Ninja depfile.  In the situation above,
AutoMoc would not re-run, because the header/source files without
Q_OBJECT macro are not contained in the depfile.

Add the relevant source files of the project to the merged depfile to
re-run AutoMoc whenever a source file changes.

Fixes: #21620