ios: load only on Mac OS

Change-Id: I2676ea1b0292c999532bf1f78c71207671e63e7f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/ios/Ios.pluginspec.in

 <plugin name=\"Ios\" version=\"$$QTCREATOR_VERSION\" compatVersion=\"$$QTCREATOR_VERSION\" experimental=\"true\">
     <vendor>Digia Plc</vendor>
     <copyright>(C) 2013 Digia Plc</copyright>
+    <platform>Mac OS.*</platform>
     <license>
 Commercial Usage