Since commit 4a9154537c (Autogen: Use cmake::IsHeader/SourceExtension
for file type detection, 2019-07-02, v3.16.0-rc1~470^2~4) we process
`.hh` files with `AUTOMOC`. However, this change can break existing
projects that do not expect the behavior. Revert it for now. It can
be restored later via a policy.
Fixes: #20101
7fa7f55230