Guard Fuchsia extensions.

So we can build for Linux without them.
The vk.xml protect="" are needed because our driver release
doesn't recognize the platform="" guard. A driver update
should fix this.

Test:
pixelbook:go/magma-tps#L1

Change-Id: I3c113b0cc38ed3812660a9836530183fa53a1f54
4 files changed