Fix http://bugs.webkit.org/show_bug.cgi?id=17001 (Build error with Gtk port on Mac...

Fix bugs.webkit.org/show_bug.cgi?id=17001 (Build error with Gtk port on Mac OS X).

Reviewed by Darin Adler.

If both XP_MACOSX and XP_UNIX are defined then X11.h and Carbon.h will both be included.
These provide conflicting definitions for a type named 'Cursor'.  As XP_UNIX is set by
the build system when targeting X11, it doesn't make sense for XP_MACOSX to also be set
in this instance.

* bindings/npapi.h: Don't define XP_MACOSX if XP_UNIX is defined.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@29806 268f45cc-cd09-0410-ab3c-d52691b4dbfc