extmod: Move fsusermount.c from stmhal for cross-port reuse.
diff --git a/py/builtin.h b/py/builtin.h
index 891b93e..5653c48 100644
--- a/py/builtin.h
+++ b/py/builtin.h
@@ -105,4 +105,7 @@
 extern const mp_obj_module_t mp_module_machine;
 extern const mp_obj_module_t mp_module_lwip;
 
+// extmod functions
+MP_DECLARE_CONST_FUN_OBJ(pyb_mount_obj);
+
 #endif // __MICROPY_INCLUDED_PY_BUILTIN_H__