build: Add get_subdir() to GeneratedList

It needs this to match the behavior of CustomTarget and
CustomTargetIndex, the later of which doesn't have a subdir attribute,
just `get_subdir()`

mesonbuild/build.py

@@ -1628,6 +1628,10 @@ class GeneratedList(HoldableObject):
     def get_extra_args(self) -> T.List[str]:
         return self.extra_args
 
+    def get_subdir(self) -> str:
+        return self.subdir
+
+
 class Executable(BuildTarget):
     known_kwargs = known_exe_kwargs