Handle relative paths correctly for files in QMAKE_BUNDLE_DATA

Partially cherry pick from f6bd02317db7beadda1237df7feddd4a780415cb
from Qt 5. The Qt 5 SHA1 changes a lot of different parts in the
XCode generator after a general qmake change caused some problems.
So we just pick the part needed to fix the handling of relative
paths used in files in QMAKE_BUNDLE_DATA.

Change-Id: I364f82a353dfa922591d07c2aa9061666b79de02
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>