mlxsw: spectrum: Add VXLAN dependency for spectrum

When VXLAN is a loadable module, MLXSW_SPECTRUM must not be built-in:

drivers/net/ethernet/mellanox/mlxsw/spectrum_switchdev.c:2547: undefined
reference to `vxlan_fdb_find_uc'

Add Kconfig dependency to enforce usable configurations.

Fixes: 1231e04f

 ("mlxsw: spectrum_switchdev: Add support for VxLAN encapsulation")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>