netfilter: nft_rt: make local functions static

Resolves warnings:
net/netfilter/nft_rt.c:26:6: warning: no previous prototype for ‘nft_rt_get_eval’ [-Wmissing-prototypes]
net/netfilter/nft_rt.c:75:5: warning: no previous prototype for ‘nft_rt_get_init’ [-Wmissing-prototypes]
net/netfilter/nft_rt.c:106:5: warning: no previous prototype for ‘nft_rt_get_dump’ [-Wmissing-prototypes]

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>