vdpasim: fix "mac_pton" undefined error

vdpasim: fix "mac_pton" undefined error

   ERROR: modpost: "mac_pton" [drivers/vdpa/vdpa_sim/vdpa_sim.ko] undefined!

mac_pton() is defined in lib/net_utils.c and is not built if NET is not set.

Select GENERIC_NET_UTILS as vdpasim doesn't depend on NET.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Link: https://lore.kernel.org/r/20201113155706.599434-1-lvivier@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested