gre: export gre_build_header() function.

This is required for ovs gre module.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/gre.h
Simple merge

diff --cc net/ipv4/gre.c
Simple merge

diff --cc net/ipv4/ip_gre.c
Simple merge