[msbuild] Add dummy CreateIpa target for Mac.

msbuild/Xamarin.Mac.Tasks/Xamarin.Mac.Common.targets

@@ -181,6 +181,9 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 		</Touch>
 	</Target>
 
+	<!-- dummy target so that we can call 'CreateIpa' from platform-agnostic code -->
+	<Target Name="CreateIpa" />
+
 	<Import Project="$(MSBuildThisFileDirectory)Xamarin.Mac.msbuild.targets" />
 
 	<Import Project="$(MSBuildThisFileDirectory)$(MSBuildThisFileName).After.targets"