Correct PostgresOrd test slightly

Ignore remarks about mutating comparisons!
...well, it still takes by-value.

pgrx-tests/tests/ui/total_ord_for_postgres_ord.rs

@@ -7,7 +7,7 @@ pub struct BrokenType {
 }
 
 impl BrokenType {
-    fn cmp(&mut self, _other: &Self) -> Anything {
+    fn cmp(self, _other: &Self) -> Anything {
         Anything::Whatever
     }
 }