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/stream_interface.h b/include/types/stream_interface.h
index 1c986ba..1af5be5 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -91,6 +91,7 @@
 
 /* Context of a running applet. */
 struct appctx {
+	enum obj_type obj_type;    /* OBJ_TYPE_APPCTX */
 	unsigned int st0;          /* CLI state for stats, session state for peers */
 	unsigned int st1;          /* prompt for stats, session error for peers */
 	unsigned int st2;          /* output state for stats, unused by peers  */