builtins: Fix missing initialize

gcc requires it even though omitting it is valid in C++20

typon/include/python/builtins.hpp

@@ -37,7 +37,7 @@
 #define FUNCTION(ret, name, args, ...)                                         \
   struct {                                                                     \
     ret operator() args const __VA_ARGS__                                            \
-  } static constexpr name;
+  } static constexpr name{};
 
 using namespace std::literals;