xbuild: include a Microsoft.Portable.Common.targets file (for F#)

F# PCL projects expect this file to exist in:
$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.Common.targets

So in order to avoid pristine F# PCL projects (created from VS) fail
the build that file has to exist, and import the Core targets like
its CSharp counterpart (Microsoft.Portable.CSharp.targets) does.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=17906