Fix the GTK+ gyp build

* JavaScriptCore.gypi: Update the source list to accurately
reflect what's in the repository and remove the offsets extractor
from the list of JavaScriptCore files. It's only used to build
the extractor binary.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142411 268f45cc-cd09-0410-ab3c-d52691b4dbfc