[DCCP]: make <linux/dccp.h> include-able from userspace

The protocol header files in <linux/foo.h> are usually structured in a
way to be included by userspace code.  The top section consists of
general protocol structure definitions, typedefs, enums - followed by
an #ifdef __KERNEL__ section.

Currently <linux/dccp.h> doesn't follow that convention and can
therefore not be used from userspace.  However, for example iptables'
libipt_dccp.c actually needs various definitions from there.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>