Fix another buildfarm issue from commit 5c279a6d35.

Discussion: https://postgr.es/m/3280724.1649340315@sss.pgh.pa.us

src/include/access/xlog_internal.h

@@ -326,6 +326,7 @@ extern void RmgrCleanup(void);
 extern void RmgrNotFound(RmgrId rmid);
 extern void RegisterCustomRmgr(RmgrId rmid, RmgrData *rmgr);
 
+#ifndef FRONTEND
 static inline bool
 RmgrIdExists(RmgrId rmid)
 {
@@ -339,6 +340,7 @@ GetRmgr(RmgrId rmid)
 		RmgrNotFound(rmid);
 	return RmgrTable[rmid];
 }
+#endif
 
 /*
  * Exported to support xlog switching from checkpointer