Fix missing return

Pointed out by Jessica in post-commit review.

Author: arichardson

pycheribuild/mtree.py

@@ -202,6 +202,7 @@ def _walk(self, top, prefix) -> "Iterator[tuple[MtreePath, list[str], list[str]]
         split = self._split_key(top)
         if split is not None:
             yield from self.children[split[0]]._walk(split[1], prefix / split[0])
+            return
         if self.entry is not None and self.entry.attributes["type"] != "dir":
             return
         files: "list[tuple[str, MtreeSubtree]]" = []