Fixed warning: "unused-function" for "RuleCompare"

I added a preprocessor if-condition to the definition of "RuleCompare"
because the only use in "gm/er.cc" is contained in such an environment in line 698

````
 #ifdef __DEBUG_ER__
static INT RuleCompare (int id, const URULE *ur, const ERULE *er)
{
  ...
}
 #endif
````

````
        #ifdef __DEBUG_ER__
  ...
  RuleCompare(ID(elem),ur,er);
  ...
        #endif
````

gm/er.cc

@@ -570,7 +570,7 @@ static HRID GetRuleID
    .n   NO:  rules are not equal
    doctext_disabled*/
 /****************************************************************************/
-
+#ifdef __DEBUG_ER__
 static INT RuleCompare (int id, const URULE *ur, const ERULE *er)
 {
   const int ns0   = NSONS_OF_RULE(ur);
@@ -627,7 +627,7 @@ static INT RuleCompare (int id, const URULE *ur, const ERULE *er)
   }
   return (YES);
 }
-
+#endif
 /****************************************************************************/
 /*doctext_disabled
     ExtractERule - extract rule from element