res_pjsip_outbound_authenticator_digest: Add missing outbound authenticator callback.

The Asterisk 13 version of the fix for outbound registration was missing
a key component that set the outbound authenticator's callback that
creates an authenticated request based on an old request. This was
picked up by some outbound registration tests failing in the testsuite.

Change-Id: I5ca9379698c606da36bc38eaffccedaf64211ce3

res/res_pjsip_outbound_authenticator_digest.c

@@ -148,6 +148,7 @@ static int digest_create_request_with_auth(const struct ast_sip_auth_vector *aut
 
 static struct ast_sip_outbound_authenticator digest_authenticator = {
 	.create_request_with_auth = digest_create_request_with_auth,
+	.create_request_with_auth_from_old = digest_create_request_with_auth_from_old,
 };
 
 static int load_module(void)