AutoMoc: Take AUTOMOC_BUILD_DIR into account in depfile

For the Ninja generator and targets that have AUTOMOC_BUILD_DIR set, the
AutoMoc target was always out of date.

That was because the depfile in the AutoMoc build directory was
referencing the wrong timestamp file:
    target_autogen/timestamp: ...dependencies...
instead of
    automoc_build_dir/timestamp: ...dependencies...

Use the relative path of the timestamp file as rule name for the
depfile.  That path is calculated with AUTOMOC_BUILD_DIR taken into
account.

Fixes: #23547
37 jobs for !7291 with fix-23547 in 78 minutes and 42 seconds (queued for 5 seconds)
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