Do not try to generate FwdHeaders when not using bundled Qt.Script
Do not try to generate FwdHeaders when not using bundled Qt.Script

Apparently, if the submodule is not present, the Rule is not invoked at
all, but it fails on Windows when submodule is there but Perl is not
found in PATH

Change-Id: I3edb4d3679f4528f05996c630c55c052c8202369
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@qt.io>

qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qtproject/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs