uml: net: vector: fix const issue

Since the constification of MAC addresses, the argument
to uml_vector_default_bpf() must be const.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: anton ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>