Add Core as dependency for the qtcreator executable in the qbs build.

Rationale: "qbs run -p qtcreator" will otherwise fail on a clean
build dir, because Creator cannot run without the Core plugin.

Change-Id: I87c16847189fc09bcc4903e865b99b6ba472f1f0
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>