Add forgot upgrade sql file

Add upgrade SQL script forgot to be included in the last commits.

Author: horiguti

Diff for: pg_store_plans--1.6--1.6.1.sql

@@ -0,0 +1,4 @@
+/* pg_store_plans/pg_store_plans--1.6--1.6.1.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION pg_store_plans" to load this file. \quit