vxlan: Remove unused argument from vxlan_build_gbp_hdr( ) and vxlan_build_gpe_hdr( )

Remove unused argument (i.e. u32 vxflags) in vxlan_build_gbp_hdr( ) and
vxlan_build_gpe_hdr( ) function arguments.

Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>