netfilter: nft_set_pipapo: remove static in nft_pipapo_get()

This has slipped through when reducing memory footprint for set
elements, remove it.

Fixes: 9dad402b

 ("netfilter: nf_tables: expose opaque set element as struct nft_elem_priv")
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>