This amends commit fc2e40e88d.
For Qt plugins, we do not add 'dlltarget' to INSTALLS but only 'target'.
In this case we must add the .dll.debug file to 'target' like we did
before fc2e40e88d.
Fixes: QTBUG-105374
Pick-to: 5.15 6.2 6.3 6.4
Change-Id: If495548dfca55d02e1c5884e03e281e27ee07ccd
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>