Add missing primary key for cache table on mysql backend

issue6840
review42731002

trytond/backend/mysql/init.sql

@@ -129,5 +129,6 @@ CREATE TABLE ir_module_dependency (
 CREATE TABLE ir_cache (
     id BIGINT AUTO_INCREMENT NOT NULL,
     name VARCHAR(255),
-    "timestamp" TIMESTAMP
+    "timestamp" TIMESTAMP,
+    PRIMARY KEY(id)
 ) ENGINE=InnoDB;