Fix function signature for Windows.

Change-Id: I604b2f755ece6a0a411562e9229a39040b3b0153
Reviewed-on: https://swiftshader-review.googlesource.com/4112
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>