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

src/backend/access/nbtree/nbtdedup.c

@@ -64,7 +64,7 @@ _bt_dedup_pass(Relation rel, Buffer buf, Relation heapRel, IndexTuple newitem,
 	BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 	Page		newpage;
 	BTDedupState state;
-	Size		pagesaving = 0;
+	Size		pagesaving PG_USED_FOR_ASSERTS_ONLY = 0;
 	bool		singlevalstrat = false;
 	int			nkeyatts = IndexRelationGetNumberOfKeyAttributes(rel);