[XFRM]: constify 'struct xfrm_type'

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index fac4f10..258d176 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -600,7 +600,7 @@
 	return err;
 }
 
-static struct xfrm_type esp_type =
+static const struct xfrm_type esp_type =
 {
 	.description	= "ESP4",
 	.owner		= THIS_MODULE,