FIX: change initObjToJSON return type
This is necessary because clang complains about the return type. There's a
call to the macro import_array() which injects a return statement into
wherever it's used.

closes #3872.

also cc @Komnomnomnom 

what do u think?
 Sounds good to me, as a fix in ultrajson I think it should be pushed upstream to them as well though. Which I can do once I've merged in their recent changes, as I might have more stuff for them.
