[GTK] [gyp] Build the final piece of WebCore
https://bugs.webkit.org/show_bug.cgi?id=113216

Reviewed by Nico Weber.

* WebCore.gyp/WebCoreGTK.gyp: Add the rest of the WebCore source files to the WebCore build target.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146800 268f45cc-cd09-0410-ab3c-d52691b4dbfc