mpls_iptunnel: add static qualifier to mpls_output

This gets rid of the following compile warn:
net/mpls/mpls_iptunnel.c:40:5: warning: no previous prototype for
mpls_output [-Wmissing-prototypes]
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Acked-by: Robert Shearman <rshearma@brocade.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mpls/mpls_iptunnel.c

@@ -37,7 +37,7 @@ static unsigned int mpls_encap_size(struct mpls_iptunnel_encap *en)
 	return en->labels * sizeof(struct mpls_shim_hdr);
 }
 
-int mpls_output(struct net *net, struct sock *sk, struct sk_buff *skb)
+static int mpls_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 {
 	struct mpls_iptunnel_encap *tun_encap_info;
 	struct mpls_shim_hdr *hdr;