Use C99 designator in the rbtree sentinel definition

This change should improve the code readability.

Discussion: https://postgr.es/m/CAGRrpzYE8-7GCoaPjOiL9T_HY605MRax-2jgTtLq236uksZ1Sw%40mail.gmail.com
Author: Steve Chavez, Alexander Korotkov
Reviewed-by: Alexander Korotkov

src/backend/lib/rbtree.c

@@ -62,7 +62,7 @@ struct RBTree
 
 static RBTNode sentinel =
 {
-	RBTBLACK, RBTNIL, RBTNIL, NULL
+	.color = RBTBLACK,.left = RBTNIL,.right = RBTNIL,.parent = NULL
 };