vxlan: Expose helper vxlan_build_gbp_hdr

The function vxlan_build_gbp_hdr will be used by other modules to build
gbp option in vxlan header according to gbp flags.

Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Gavi Teitz <gavi@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Acked-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>