Qbs: Fix qbs install step

Fix setting the installroot of the qbs install step.

Task-number: QBS-353
Task-number: QBS-354
Change-Id: Ia99abd3044a9ec90c67de3060d5895ae0b6b8652
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/qbsprojectmanager/qbsinstallstep.cpp

@@ -229,7 +229,7 @@ void QbsInstallStep::setInstallRoot(const QString &ir)
 {
     if (m_qbsInstallOptions.installRoot() == ir)
         return;
-    m_qbsInstallOptions.installRoot() = ir;
+    m_qbsInstallOptions.setInstallRoot(ir);
     emit changed();
 }