generator: incorrect code generated for instance category properties

Test case:

[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.