[T4] Fix Initialize visibility. Fixes #24659.

Otherwise this happens:

/var/folders/_n/1849dyl138g79qwz2fr8h74m0000gn/T/6a2cf289/1fe4247d.0.cs(944,33): ERROR `Microsoft.VisualStudio.TextTemplating.GeneratedTextTransformation4bc89c69.Initialize()': cannot change access modifiers when overriding `public' inherited member `Microsoft.VisualStudio.TextTemplating.TextTransformation.Initialize()'
(0,0): ERROR /Applications/Xamarin Studio.app/Contents/Resources/lib/monodevelop/AddIns/MonoDevelop.TextTemplating/Mono.TextTemplating.dll (Location of the symbol related to previous error)

https://bugzilla.xamarin.com/show_bug.cgi?id=24659