Fix macOS build for Qt 5.6.1
Fix macOS build for Qt 5.6.1

Use the macx qmake scope instead of the macos scope that was introduced
somewhen after Qt 5.6.1.

Change-Id: Ie70da955373e5db9b910eeb26abe0decda385725
Reviewed-by: Christian Kandeler <3aa99dcdd3f0cac61fb81c2a11771c0cc2497607@qt.io>

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