The .def file is already in sources, which means GN will already add
this flag automatically (cf `gn help sources`).

No intended behavior change.

Bug: none
Change-Id: Ic8204bbdd9da47f3731d48e62ccfeb880ac9eddc
Reviewed-on: https://swiftshader-review.googlesource.com/c/23528Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nico Weber <thakis@chromium.org>