gnextstepsettingsbackend: Use same token for function-call and its availability

Availability of the g_nextstep_settings_backend_get_type() prototype
is controlled by HAVE_COCOA in gsettingsbackendinternal.h and the
actual implemenation by OS_COCOA in Makefile.am. Therefore, the
giomodule.c call to that function should also be protected by a COCOA
token rather than an CARBON token (cocoa and carbon are independent
autoconf tests).

https://bugzilla.gnome.org/show_bug.cgi?id=731425