cloud_storage: recursive_directory_walker eliminate a sstring copy

(cherry picked from commit 3fedeb3)

src/v/cloud_storage/recursive_directory_walker.cc

@@ -74,7 +74,7 @@ struct walk_accumulator {
     bool empty() const { return dirlist.empty(); }
 
     ss::sstring pop() {
-        auto r = dirlist.back();
+        auto r = std::move(dirlist.back());
         dirlist.pop_back();
         return r;
     }