certificate: Return signature scheme and parameters from issued_by() method

This also required some include restructuring (avoid including library.h
in headers) to avoid unresolvable circular dependencies.

src/libstrongswan/plugins/pgp/pgp_cert.c

@@ -114,7 +114,7 @@ METHOD(certificate_t, has_issuer, id_match_t,
 }
 
 METHOD(certificate_t, issued_by,bool,
-	private_pgp_cert_t *this, certificate_t *issuer, signature_scheme_t *scheme)
+	private_pgp_cert_t *this, certificate_t *issuer, signature_params_t **scheme)
 {
 	/* TODO: check signature blobs for a valid signature */
 	return FALSE;