FreeTDM: Add span iterator

Part of my ongoing effort to split ftdm_io.c into more manageable pieces.

This change (along with others in the future) allows decoupling of the
span registry and its users, in preparation of moving all span related
functions and data structures into ftdm_span.c.

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

libs/freetdm/src/include/private/ftdm_types.h

@@ -360,6 +360,7 @@ typedef ftdm_status_t (*ftdm_channel_sig_dtmf_t)(ftdm_channel_t *ftdmchan, const
 typedef enum {
 	FTDM_ITERATOR_VARS = 1,
 	FTDM_ITERATOR_CHANS,
+	FTDM_ITERATOR_SPANS,
 } ftdm_iterator_type_t;
 
 struct ftdm_iterator {