util: Make dtmf tone checking function public

Moves the function out of calls-call.c into util.c

src/util.h

@@ -141,6 +141,8 @@ gboolean calls_find_in_store (GListModel *list,
 const char* get_protocol_from_address (const char *target);
 const char* get_protocol_from_address_with_fallback (const char *target);
 
+gboolean dtmf_tone_key_is_valid (char key);
+
 G_END_DECLS
 
 #endif /* CALLS__UTIL_H__ */