[IPV6] UDPLITE: Sparse: Declare non-static symbols in header.

Fix the following sparse warnings:
| net/ipv6/udplite.c:45:14: warning: symbol 'udplitev6_prot' was not declared. Should it be static?
| net/ipv6/udplite.c:80:12: warning: symbol 'udplitev6_init' was not declared. Should it be static?
| net/ipv6/udplite.c:99:6: warning: symbol 'udplitev6_exit' was not declared. Should it be static?

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>