Windows: disable the (currently broken) Windows platform plugin

This plugin currently refers to some API which does not exist
(focusObject).  It has no chance to compile for anyone.  Disable it
until fixed.

Change-Id: I3b74b61bf5f73ba06ff73375b233a4e4ca6c1678
Reviewed-on: http://codereview.qt-project.org/4843
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Kalle Lehtonen <kalle.ju.lehtonen@nokia.com>

src/plugins/platforms/platforms.pro

@@ -14,4 +14,7 @@ mac {
     SUBDIRS += cocoa
 }
 
-win32: SUBDIRS += windows
+win32 {
+    warning(Windows platform plugin is currently not working)
+    #SUBDIRS += windows
+}