net: Update entry in af_family_clock_key_strings

In the merge phase of the CAN subsystem the 
af_family_clock_key_strings[] have been added to sock.c in commit 
443aef0e

 
(lockdep: fixup sk_callback_lock annotation). This trivial patch adds 
the missing name for address family 29 (AF_CAN).

Signed-off-by: Oliver Hartkopp <oliver@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>