netfilter: uapi: Avoid undefined left-shift in xt_sctp.h

With 'bytes(__u32)' being 32, a left-shift of 31 may happen which is
undefined for the signed 32-bit value 1. Avoid this by declaring 1 as
unsigned.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>