Fix failing -D HASH_STATISTICS builds

This seems to have been broken for a few years by cc5ef90ed.

Author: Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/OSCPR01MB14966E11EEFB37D7857FCEDB7F535A@OSCPR01MB14966.jpnprd01.prod.outlook.com
Backpatch-through: 17

src/backend/utils/hash/dynahash.c

@@ -1158,6 +1158,8 @@ hash_update_hash_key(HTAB *hashp,
 	HashCompareFunc match;
 
 #ifdef HASH_STATISTICS
+	HASHHDR    *hctl = hashp->hctl;
+
 	hash_accesses++;
 	hctl->accesses++;
 #endif