vxlan: fix multiple inclusion of vxlan.h

The vxlan_get_sk_family inline function was added after the last #endif,
making multiple inclusion of net/vxlan.h fail. Move it to the proper place.

Reported-by: Mark Rustad <mark.d.rustad@intel.com>
Fixes: 705cc62f ("vxlan: provide access function for vxlan socket address family")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>