To prevent Chromium from building the "swiftshader_subzero"
source set when building all, the source set is now located
within an "if (use_swiftshader_with_subzero)" statement.

Change-Id: I4363487103ec8cd6856b1afb0dd861146a3e56bd
Reviewed-on: https://swiftshader-review.googlesource.com/8728Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>