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/x509/x509_ocsp_request.c

@@ -372,7 +372,7 @@ METHOD(certificate_t, has_issuer, id_match_t,
 
 METHOD(certificate_t, issued_by, bool,
 	private_x509_ocsp_request_t *this, certificate_t *issuer,
-	signature_scheme_t *scheme)
+	signature_params_t **scheme)
 {
 	DBG1(DBG_LIB, "OCSP request validation not implemented!");
 	return FALSE;