esp8266,minimal,pic16bit: Use size_t for mp_builtin_open argument.

py/builtin.h declares mp_builtin_open with the first argument of type
size_t.  Make all implementations conform to this declaration.
diff --git a/esp8266/main.c b/esp8266/main.c
index e3188df..43b8375 100644
--- a/esp8266/main.c
+++ b/esp8266/main.c
@@ -122,7 +122,7 @@
     return MP_IMPORT_STAT_NO_EXIST;
 }
 
-mp_obj_t mp_builtin_open(uint n_args, const mp_obj_t *args, mp_map_t *kwargs) {
+mp_obj_t mp_builtin_open(size_t n_args, const mp_obj_t *args, mp_map_t *kwargs) {
     return mp_const_none;
 }
 MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_open_obj, 1, mp_builtin_open);