Core: Fix qbs build.

Change-Id: Idcefac332e3504fee704bb2dc3328bf50c7bd0f2
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/coreplugin/coreplugin.qbs

@@ -100,6 +100,7 @@ QtcPlugin {
             "variablemanager.cpp", "variablemanager.h",
             "vcsmanager.cpp", "vcsmanager.h",
             "versiondialog.cpp", "versiondialog.h",
+            "windowsupport.cpp", "windowsupport.h"
         ]
     }