Fix macro use in os.hpp

rt/include/python/os.hpp

@@ -22,7 +22,7 @@ int no_special(const struct dirent *d) {
 namespace py_os {
 struct os_t {
   FUNCTION(
-      auto, fsdecode, (std::string s) { return s; })
+      auto, fsdecode, (std::string s), { return s; })
 
   struct Stat_Result_s {
     struct py_type {