py: Clean up instantiation of dupterm object.

To comply with already established scheme for extmod's.
diff --git a/py/mpstate.h b/py/mpstate.h
index 012b0ef..a990713 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -127,6 +127,11 @@
     MICROPY_PORT_ROOT_POINTERS
 
     // root pointers for extmod
+
+    #if MICROPY_PY_OS_DUPTERM
+    mp_obj_t term_obj;
+    #endif
+
     #if MICROPY_PY_LWIP_SLIP
     mp_obj_t lwip_slip_stream;
     #endif