mypy: add annotation for object attribute used as a private cache via hasattr

If it exists, we want to know it has the right type when we use it in
ninjabackend.py.

mesonbuild/build.py

@@ -721,6 +721,9 @@ class BuildTarget(Target):
         ('cpp', 'cuda'),
     ])
 
+    # This is used by the backend to cache complex computation.
+    cached_generated_headers: T.Optional[T.List[FileOrString]]
+
     def __init__(
             self,
             name: str,