Suppress "variable 'pagesaving' set but not used" warning.

With asserts disabled, late-model clang notices that this variable
is incremented but never otherwise read.

Discussion: https://postgr.es/m/3171401.1649275153@sss.pgh.pa.us

Author: tglsfdc

src/backend/access/nbtree/nbtdedup.c

@@ -65,7 +65,7 @@ _bt_dedup_one_page(Relation rel, Buffer buf, Relation heapRel,
 	OffsetNumber deletable[MaxIndexTuplesPerPage];
 	BTDedupState state;
 	int			ndeletable = 0;
-	Size		pagesaving = 0;
+	Size		pagesaving PG_USED_FOR_ASSERTS_ONLY = 0;
 	bool		singlevalstrat = false;
 	int			nkeyatts = IndexRelationGetNumberOfKeyAttributes(rel);