Fix svn-clean script for symlinks to directories

git-svn-id: https://openmodelica.org/svn/OpenModelica/trunk@18199 f25d12d1-65f4-0310-ae8a-bbce733d8d8e

svn-clean.py

@@ -5,7 +5,7 @@
 import re
 
 def removeall(path):
-    if not os.path.isdir(path):
+    if os.path.islink(path) or not os.path.isdir(path):
         os.remove(path)
         return
     files=os.listdir(path)