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>