QmlDesigner.desktopPlugin: fix for plugindestdir

Change-Id: I31a725f4398345ae850361faa364ecdcb7ca7b2d
Reviewed-on: http://codereview.qt-project.org/5703

Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>

src/plugins/qmldesigner/desktopplugin/desktopplugin.pro

 include(../../../../qtcreator.pri)
 include(../../../private_headers.pri)
 include(desktopplugin.pri)
-include(plugindestdir.pri)
+include(../plugindestdir.pri)

src/plugins/qmldesigner/desktopplugin/plugindestdir.pri

-macx {
-  DESTDIR = $$IDE_LIBRARY_PATH/QmlDesigner
-} else {
-  DESTDIR = $$IDE_BUILD_TREE/lib/qmldesigner
-}