netfilter: ebtables: use __u64 from linux/types.h

Fixes userspace compilation error:

linux/netfilter_bridge/ebtables.h:38:2: error: unknown type name ‘uint64_t’

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>