platform: generic-arm64: Guard vsock-rust

Only add vsock-rust dependency if we're building Trusty for a regular or
protected VM (-virt builds) where ARM64_BOOT_PROTOCOL is set to X0_DTB.

Bug: None
Test: Presubmit
Change-Id: I5bbb7e79528d211cd3e850c03914050cf38db313
2 files changed