binding ruby: Suppress warnings

    rb-milter-client-context.c: In function 'get_mail_transaction_shelf':
    rb-milter-client-context.c:296:58: warning: passing argument 3 of 'milter_client_context_mail_transaction_shelf_foreach' makes pointer from integer without a cast [enabled by default]
                                                              rb_hash);
                                                              ^
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