xcode: Drop use of weak_import on enumerators.

GCC doesn't support attributes on enumerators, which could pose a
problem but fortunately not in this case. Here a
__attribute__((weak_import)) is used, which doesn't make much sense for
enumerators anyway (noone will die because the corresponding enumerator
won't be referenced either in older OS X versions).

Signed-off-by: aszlig <aszlig@redmoonstudios.org>