docs/conf.py: add function attribute '__fix_address' to conf.py

Stephen Rothwell reported htmldocs warning when merging net-next:

Documentation/networking/kapi:26: net/core/skbuff.c:780: WARNING: Error in declarator or parameters
Invalid C declaration: Expecting "(" in parameters. [error at 19]
  void __fix_address kfree_skb_reason (struct sk_buff *skb, enum skb_drop_reason reason)
  -------------------^

Add __fix_address keyword to c_id_attributes array in conf.py to fix
the warning.

Link: https://lore.kernel.org/linux-next/20220825154105.534d78ab@canb.auug.org.au/
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>