modules/gnome: fix gen_marshall annotation
@@ -149,7 +149,7 @@ if T.TYPE_CHECKING:
build_by_default: T.Optional[bool]
depend_files: T.List[mesonlib.File]
extra_args: T.List[str]
- install_dir: T.Union[str, bool]
+ install_dir: T.Optional[str]
install_header: bool
internal: bool
nostdinc: bool
