res_pjsip: Fix warning by deferring implicit type cast.

Mac doesn't like the comparison of -1 to an enum, so store the result of
ast_sip_str_to_dtmf to an int so we can check for the negative return
value.  ast_sip_str_to_dtmf returns an int so this is only delaying the
implicit type cast.

Change-Id: I0c262c1719ee951aae1f437d733a301cf5f8ad29

res/res_pjsip/pjsip_configuration.c

@@ -368,7 +368,7 @@ static int contact_acl_to_str(const void *obj, const intptr_t *args, char **buf)
 static int dtmf_handler(const struct aco_option *opt, struct ast_variable *var, void *obj)
 {
 	struct ast_sip_endpoint *endpoint = obj;
-	enum ast_sip_dtmf_mode dtmf = ast_sip_str_to_dtmf(var->value);
+	int dtmf = ast_sip_str_to_dtmf(var->value);
 
 	if (dtmf == -1) {
 		return -1;