[DFS] Export dfs_subdir/dfs_normalize_path symbol to module.

components/dfs/src/dfs.c

@@ -284,6 +284,7 @@ const char *dfs_subdir(const char *directory, const char *filename)
 
     return dir;
 }
+RTM_EXPORT(dfs_subdir);
 
 /**
  * this function will normalize a path according to specified parent directory
@@ -410,5 +411,7 @@ up_one:
 
     return fullpath;
 }
+RTM_EXPORT(dfs_normalize_path);
+
 /*@}*/