net/core/sock.c: add missing VSOCK string in af_family_*_key_strings

The three arrays of strings: af_family_key_strings,
af_family_slock_key_strings and af_family_clock_key_strings have not
VSOCK's string

Signed-off-by: Federico Vaga <federico.vaga@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>