[XFRM]: unexport xfrm_state_mtu

This patch removes the unused EXPORT_SYMBOL(xfrm_state_mtu).
Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/xfrm/xfrm_state.c

@@ -1164,8 +1164,6 @@ int xfrm_state_mtu(struct xfrm_state *x, int mtu)
 	return res;
 }
 
-EXPORT_SYMBOL(xfrm_state_mtu);
-
 int xfrm_init_state(struct xfrm_state *x)
 {
 	struct xfrm_state_afinfo *afinfo;