netdevsim: don't compile BPF code if syscall not enabled

We should not compile netdevsim/bpf.c if BPF syscall is not
enabled.  Otherwise bpf core would have to provide wrappers
for all functions offload drivers may call, even though
system will never see a BPF object.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>