Linux: Adds missing abc.Iterable implementation

`hlist_head` is missing an implementation for
`collections.abc.Iterable`, resulting in a crash at runtime. This fixes
the issue by providing the required `__iter__` implementation for
`hlist_head`.

volatility3/framework/symbols/linux/extensions/__init__.py

@@ -1037,6 +1037,9 @@ def to_list(
 
             current = current.next
 
+    def __iter__(self) -> Iterator[interfaces.objects.ObjectInterface]:
+        return self.to_list(self.vol.parent.vol.type_name, self.vol.member_name)
+
 
 class files_struct(objects.StructType):
     def get_fds(self) -> interfaces.objects.ObjectInterface: