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

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

py/modsys.c

@@ -39,9 +39,9 @@
 /// \module sys - system specific functions
 
 // defined per port; type of these is irrelevant, just need pointer
-extern mp_uint_t mp_sys_stdin_obj;
-extern mp_uint_t mp_sys_stdout_obj;
-extern mp_uint_t mp_sys_stderr_obj;
+extern struct _mp_dummy_t mp_sys_stdin_obj;
+extern struct _mp_dummy_t mp_sys_stdout_obj;
+extern struct _mp_dummy_t mp_sys_stderr_obj;
 
 /// \constant version - Python language version that this implementation conforms to, as a string
 STATIC const MP_DEFINE_STR_OBJ(version_obj, "3.4.0");

py/obj.c

@@ -74,7 +74,7 @@ void mp_obj_print_helper(void (*print)(void *env, const char *fmt, ...), void *e
 void mp_obj_print(mp_obj_t o_in, mp_print_kind_t kind) {
 #if MICROPY_PY_IO
     // defined per port; type of these is irrelevant, just need pointer
-    extern mp_uint_t mp_sys_stdout_obj;
+    extern struct _mp_dummy_t mp_sys_stdout_obj;
     pfenv_t pfenv;
     pfenv.data = &mp_sys_stdout_obj;
     pfenv.print_strn = (void (*)(void *, const char *, mp_uint_t))mp_stream_write;