BPathFinder: include StringList.h for convenience.

* You cannot do anything with a path finder object without a string
  list, so why go through all the hassle?

headers/os/storage/PathFinder.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013, Haiku, Inc. All Rights Reserved.
+ * Copyright 2013-2015, Haiku, Inc. All Rights Reserved.
  * Distributed under the terms of the MIT License.
  */
 #ifndef _PATH_FINDER_H
@@ -7,11 +7,10 @@
 
 
 #include <FindDirectory.h>
-#include <String.h>
+#include <StringList.h>
 
 
 class BPath;
-class BStringList;
 struct entry_ref;