xtables-restore: Treat struct nft_xt_restore_parse as const

This structure contains restore parser configuration, parser is not
supposed to alter it.

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