be2net: fix build dependency on VxLAN

Introduce a CONFIG_BE2NET_VXLAN define to control be2net's build
dependency on the VXLAN driver.

Without this fix, the kernel build fails when VxLAN driver is
selected to be built as a module while be2net is built-in.

fixes: c9c47142

 ("be2net: csum, tso and rss steering offload support for VxLAN")

Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>