[DCCP] ipv4: make struct dccp_v4_prot static

There's no reason for struct dccp_v4_prot being global.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>