Fix GN for rtc_base_approved target.

In https://webrtc-codereview.appspot.com/22649004
a new target was introduced that duplicated some
source files, breaking the bots in
http://build.chromium.org/p/chromium.webrtc.fyi/waterfall

This updates the GN config to also remove them from
the target where they were moved from in base.gyp.

BUG=3806
TESTED=Trybots + Running GN in a Chromium checkout with
src/third_party/webrtc symlinked to the WebRTC checkout
with this CL applied + passing compile step.

R=perkj@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/23669004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7192 4adac7df-926f-26a2-2b94-8c16560cd09d