gtp: constify nla_policy

The netlink policy structure can be constant like other
drivers.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gtp.c

@@ -1255,7 +1255,7 @@ out:
 	return skb->len;
 }
 
-static struct nla_policy gtp_genl_policy[GTPA_MAX + 1] = {
+static const struct nla_policy gtp_genl_policy[GTPA_MAX + 1] = {
 	[GTPA_LINK]		= { .type = NLA_U32, },
 	[GTPA_VERSION]		= { .type = NLA_U32, },
 	[GTPA_TID]		= { .type = NLA_U64, },