add missing migration table index (#26576)

htdocs/install/mysql/migration/18.0.0-19.0.0.sql

@@ -189,3 +189,5 @@ CREATE TABLE llx_mrp_production_extrafields
     fk_object                 integer NOT NULL,
     import_key                varchar(14)                          		-- import key
 ) ENGINE=innodb;
+
+ALTER TABLE llx_mrp_production_extrafields ADD INDEX idx_mrp_production_fk_object(fk_object);