Fix: xcode-ninja should copy the product extension to the wrapper project

Two targets may differ in just the product extension. To avoid name
clashes in this situation, the xcode-ninja generator needs to copy this
extension when generating targets for the wrapper project.

Patch by Tobias Hahn <tobias.hahn@ableton.com>.

BUG=gyp:450
TEST=./gyptest.py -f xcode-ninja test/mac/gyptest-loadable-module-bundle-product-extension.py
R=justincohen@chromium.org, sdefresne@chromium.org, justincohen

Review URL: https://codereview.chromium.org/516953002

git-svn-id: http://gyp.googlecode.com/svn/trunk@2019 78cadc50-ecff-11dd-a971-7dbc132099af