py: Use a dummy type for referring to extern structs
Fixes msvc linker warnings about mismatching sizes between the mp_obj_fdfile_t
struct defined in file.c and the mp_uint_t declarations found in modsys.c and modbuiltins.c
diff --git a/py/modbuiltins.c b/py/modbuiltins.c
index 0985e25..04141ed 100644
--- a/py/modbuiltins.c
+++ b/py/modbuiltins.c
@@ -42,7 +42,7 @@
#endif
#if MICROPY_PY_IO
-extern mp_uint_t mp_sys_stdout_obj; // type is irrelevant, just need pointer
+extern struct _mp_dummy_t mp_sys_stdout_obj; // type is irrelevant, just need pointer
#endif
// args[0] is function from class body