Add .def file to implicit deps

This is so that when the .def file is modified, a relink will be triggered.

R=scottmg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/18714005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1659 78cadc50-ecff-11dd-a971-7dbc132099af