rtnetlink: avoid 0 sized arrays

Arrays (when not in a struct) "shall have a value greater than zero".

GCC complains when it's not the case here.

Fixes: ba7d49b1

 ("rtnetlink: provide api for getting and setting slave info")
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>