Signal is a keyword for MySQL 5.5

issue2037
review30006

trytond/backend/mysql/init.sql

@@ -121,7 +121,7 @@ CREATE TABLE wkf_transition (
     act_to BIGINT,
     `condition` VARCHAR(255),
     trigger_expr_id VARCHAR(255),
-    signal VARCHAR(255),
+    `signal` VARCHAR(255),
     `group` BIGINT,
     PRIMARY KEY(id),
     CONSTRAINT wkf_transition_act_from_fkey FOREIGN KEY (act_from) REFERENCES wkf_activity (id) ON DELETE CASCADE,