generator: incorrect code generated for instance category properties

Test case:

[Category]
[BaseType (typeof(NSObject))]
interface SomeCategory
{

	[Export ("someValue", ArgumentSemantic.Strong)]
	string SomeValue { get; }

}


generates code that doesn't compile:

> obj/Debug/ios/SomeCategory.g.cs(49,24): error CS0708: 'SomeCategory.SomeValue': cannot declare instance members in a static class

it seems that we should generate these as static (we generate methods as static), but for some reason we aren't.


Reference: https://bugzilla.xamarin.com/show_bug.cgi?id=57095