[uart] Fix uart driver & testapp j6::thread usage

The prior change to j6::thread allowing arguments did not test drv.uart
or testapp. Oops.

src/user/drv.uart/main.cpp

@@ -81,7 +81,7 @@ channel_pump_loop()
 }
 
 void
-pump_proc()
+pump_proc(void *)
 {
     j6_process_exit(channel_pump_loop());
 }