Mark StructNdOptPref64#prefix @NonNull.

The prefix can never be null. Mark it as such to make it clearer
that no null checks for it are needed.

Bug: 153694684
Test: atest NetworkStackTests:NduseroptMessageTest NetworkStackTests:StructNdOptPref64Test
Change-Id: Ica4f6f7545e63e0665c2a4ae4de8323f3fd93e1c