ip_gre: Add IPPROTO_GRE to flowi in ipgre_tunnel_xmit

Commit 5811662b

 ("net: use the macros
defined for the members of flowi") accidentally removed the setting of
IPPROTO_GRE from the struct flowi in ipgre_tunnel_xmit. This patch
restores it.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>