modules/icestorm: fix type annotations

This is also due to the incorrect annotations of state.find_program()

mesonbuild/modules/icestorm.py

@@ -40,7 +40,7 @@ class IceStormModule(ExtensionModule):
 
     def __init__(self, interpreter: Interpreter) -> None:
         super().__init__(interpreter)
-        self.tools: T.Dict[str, ExternalProgram] = {}
+        self.tools: T.Dict[str, T.Union[ExternalProgram, build.Executable]] = {}
         self.methods.update({
             'project': self.project,
         })