The `ErrTimeout` type was promoted to the OS-common .go file, but I forgot to remove it from the unix .go file.

Change-Id: Ic30675bd7bc1c5289080c45e21ce9cf19966d3e8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40708
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ben Clayton <bclayton@google.com>