Uses/qmake.mk: fix build WITH_DEBUG

When setting WITH_DEBUG, separate debuginfo was not created,
although the plists for most Qt5 ports list separate debuginfo.
Take a suggestion from rakuco@ and run with it. I rebuilt all the
ports with USES=qmake and none had issues with separate debuginfo.

No general PORTREVISION bump because the packages in most cases
do not change with default options, and WITH_DEBUG would have
failed to package anyway.

PR: 240494
Reported by: jbeich