stubgenc: small fix in get_default_function_sig (#19822)

This small change fixes a crash in the case when a function arg has both
a default value and a non-string type annotation.

Here is an example:
```
def f(i: int = 0): pass
```

Author: ilya-pevzner

mypy/stubgenc.py

@@ -322,7 +322,7 @@ def add_args(
                 default_value = get_default_value(i, arg)
                 if default_value is not _Missing.VALUE:
                     if arg in annotations:
-                        argtype = annotations[arg]
+                        argtype = get_annotation(arg)
                     else:
                         argtype = self.get_type_annotation(default_value)
                         if argtype == "None":