Fixes build failures seen here:
https://travis-ci.org/paulthomson/build-swiftshader/jobs/565093397

Change-Id: Ie03b1dbbbbbcc586dc4dfc7eba85e0e225012a16
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34456Reviewed-by: Paul Thomson <paulthomson@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>