MINOR: obj: introduce a new type appctx

The object type was added to "struct appctx". The purpose will be
to identify an appctx when the applet context is detached from the
stream interface. For now, it's still attached, so this patch only
adds the new type and does not replace its use.
diff --git a/include/types/obj_type.h b/include/types/obj_type.h
index 88d8973..09eaf43 100644
--- a/include/types/obj_type.h
+++ b/include/types/obj_type.h
@@ -37,6 +37,7 @@
 	OBJ_TYPE_PROXY,        /* object is a struct proxy */
 	OBJ_TYPE_SERVER,       /* object is a struct server */
 	OBJ_TYPE_APPLET,       /* object is a struct si_applet */
+	OBJ_TYPE_APPCTX,       /* object is a struct appctx */
 	OBJ_TYPE_CONN,         /* object is a struct connection */
 	OBJ_TYPE_ENTRIES       /* last one : number of entries */
 };