Xcode: Add xcdatamodeld = wrapper.xcdatamodeld in extension_map 

.xcdatamodeld documents are used for Model File Format. 
https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreDataVersioning/Articles/vmModelFormat.html 

Note that .xcdatamodel has already been supported since gyp r1529. 

BUG=303

Patch from Roberto Cosenza <robcos@google.com>
via Yohei Yukawa <yukawa@chromium.org>

Review URL: https://codereview.chromium.org/351223002/


git-svn-id: http://gyp.googlecode.com/svn/trunk@1948 78cadc50-ecff-11dd-a971-7dbc132099af