From `gn help sources`:

  As a special case, a file ending in ".def" will be treated as a Windows
  module definition file. It will be appended to the link line with a
  preceding "/DEF:" string.

This makes the code slightly shorter, and also gives ninja a chance of
re-running the link when the .def file changes since it now knows about
this file.

No intended behavior change.

Change-Id: I5fecb22752508880b726482f7c3da5a75180e446
Reviewed-on: https://chromium-review.googlesource.com/c/1396499Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>