[Rspamd] Increase spam scores for SPF failures

data/conf/rspamd/local.d/metrics.conf

@@ -38,3 +38,6 @@ symbol "SPOOFED_SENDER" {
 	description = "Sender is not authenticated but part of mailcow managed domains";
 	score = 1.0;
 }
+symbol "R_SPF_FAIL" {
+  score = 4.0;
+}