add model_pdf fields for edition feature

htdocs/install/mysql/tables/llx_product.sql

@@ -81,6 +81,7 @@ create table llx_product
   finished					tinyint      DEFAULT NULL,		-- 1=manufactured product, 0=matiere premiere
   hidden					tinyint      DEFAULT 0,			-- Not used. Deprecated.
   import_key				varchar(14),					-- Import key
+  model_pdf				varchar(255),           -- model save dodument used
   fk_price_expression integer,                     -- Link to the rule for dynamic price calculation
   desiredstock              integer      DEFAULT 0,
   fk_unit					integer      DEFAULT NULL,