MINOR: checks/obj_type: Add a new object type for checks

An object type is now affected to the check structure.
diff --git a/include/types/checks.h b/include/types/checks.h
index f52e4a4..d1a53ed 100644
--- a/include/types/checks.h
+++ b/include/types/checks.h
@@ -158,6 +158,7 @@
 };
 
 struct check {
+	enum obj_type obj_type;                 /* object type == OBJ_TYPE_CHECK */
 	struct session *sess;			/* Health check session. */
 	struct vars vars;			/* Health check dynamic variables. */
 	struct xprt_ops *xprt;			/* transport layer operations for health checks */