Deniz Bahadir authored Nov 13, 2020 and Brad King committed Nov 16, 2020

The PCH header file itself is written by CMake and is not generated by
part of the buildsystem.  Therefore the `GENERATED` property is not
appropriate to set because the file is always present before the build
starts.  We already do not mark the PCH source file as `GENERATED`
either.

This is a preparation for solving #18399, which will no longer allow to
unset the `GENERATED` property from a source-file once it was set.

Fixes: #21437


Signed-off-by: Deniz Bahadir <dbahadir@benocs.com>