windeployqt: Concatenate qmlimportscanner results.

Fix up f5b851f83cfb315bbc87ef878bde0b2573f225cf introducing
multiple values for the -qmldir option.

Change-Id: Ib199105776d42b6eaa81003ed5682b2272a19a53
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>