fix(header): add compose return type

Author: TomJGooding

src/textual/widgets/_header.py

@@ -6,7 +6,7 @@
 
 from rich.text import Text
 
-from textual.app import RenderResult
+from textual.app import ComposeResult, RenderResult
 from textual.dom import NoScreen
 from textual.events import Click, Mount
 from textual.reactive import Reactive
@@ -181,7 +181,7 @@ def __init__(
  if time_format is not None:
  self.time_format = time_format
 
- def compose(self):
+ def compose(self) -> ComposeResult:
  yield HeaderIcon().data_bind(Header.icon)
  yield HeaderTitle()
  yield (