fix pg migration script

htdocs/install/mysql/migration/3.8.0-3.9.0.sql

@@ -318,7 +318,8 @@ ALTER TABLE llx_categorie_project ADD CONSTRAINT fk_categorie_project_fk_project
 
 
 ALTER TABLE llx_c_tva ADD COLUMN code varchar(10) DEFAULT '' after fk_pays;
-DROP INDEX uk_c_tva_id ON llx_c_tva;
+-- VMYSQL4.0 DROP INDEX uk_c_tva_id ON llx_c_tva;
+-- VPGSQL8.0 DROP INDEX uk_c_tva_id;
 ALTER TABLE llx_c_tva ADD UNIQUE INDEX uk_c_tva_id (fk_pays, code, taux, recuperableonly);
 
 -- Regions Bolivia (id country=52)