[objcgen] fix error MM0073 for Xamarin.Mac 4.8~ (#733)
[objcgen] fix error MM0073 for Xamarin.Mac 4.8~ (#733)

- Xamarin.Mac 4.8 only supports macOS 10.9 (Mavericks) or higher.
fixes https://github.com/mono/Embeddinator-4000/issues/732
jonathanpeppers/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,mono/Embeddinator-4000,mono/Embeddinator-4000,jonathanpeppers/Embeddinator-4000,mono/Embeddinator-4000