chan_sip.c: Fix __sip_reliable_xmit build error

Fixes #954

channels/chan_sip.c

@@ -1202,7 +1202,7 @@ static int get_address_family_filter(unsigned int transport);
 /*--- Transmitting responses and requests */
 static int sipsock_read(int *id, int fd, short events, void *ignore);
 static int __sip_xmit(struct sip_pvt *p, struct ast_str *data);
-static int __sip_reliable_xmit(struct sip_pvt *p, uint32_t seqno, int resp, struct ast_str *data, int fatal, int sipmethod);
+static enum sip_result __sip_reliable_xmit(struct sip_pvt *p, uint32_t seqno, int resp, struct ast_str *data, int fatal, int sipmethod);
 static void add_cc_call_info_to_response(struct sip_pvt *p, struct sip_request *resp);
 static int __transmit_response(struct sip_pvt *p, const char *msg, const struct sip_request *req, enum xmittype reliable);
 static int retrans_pkt(const void *data);