make 1st argument of $$resolve_depends a variable

make 1st argument of $$resolve_depends a variable

this is braindead, but it's consistent with the rest of qmake and more
performant. and the argument error message claimed it already anyway.

Change-Id: I973368acc6ffbff17107085ccd68b0334cc3e681
Reviewed-on: http://codereview.qt.nokia.com/1436
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>