vulkan-samples: Disable PCH for reproducibility

We're still seeing reproducibility issues on the autobuilder with this
recipe. I was able to make the output "match" by rebuilding the PCH and
then rebuilding the binary objects, proving the PCH isn't deterministic.

Disable PCH until we can get to the bottom of why that may be.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>