net: mpls: make function ipgre_mpls_encap_hlen static

The function ipgre_mpls_encap_hlen is local to the source and
does not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'ipgre_mpls_encap_hlen' was not declared. Should it be static?

Fixes: bdc47641 ("ip_tunnel: add mpls over gre support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>