netfilter: xtables: add missing const qualifier to xt_tgchk_param

When entryinfo was a standalone parameter to functions, it used to be
"const void *". Put the const back in.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>