[fix] kernel-doc comandline, default of --known-attrs is an empty list

Tested by::

   kernel-doc --exported \
              --symbols-exported-identifiers API_EXPORTED \
	      --symbols-exported-method=attribute \
	      ./docs/linuxdoc-howto/all-in-a-tumble.c

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

linuxdoc/kernel_doc.py

@@ -502,7 +502,7 @@ def main():
 
     CLI.add_argument(
         "--known-attrs"
-        , default = ""
+        , default = []
         , nargs   = "+"
         , help    = ("provides a list of known attributes that has to be"
                      " hidden when displaying function prototypes"))