selftests, bpf: test_tc_tunnel.sh nc: Cannot use -p and -l

When run test_tc_tunnel.sh, it complains following error

ipip
encap 192.168.1.1 to 192.168.1.2, type ipip, mac none len 100
test basic connectivity
nc: cannot use -p and -l

nc man page has:

     -l  Listen for an incoming connection rather than initiating
         a connection to a remote host.Cannot be used together with
         any of the options -psxz. Additionally, any timeouts specified
         with the -w option are ignored.

Correct nc in server_listen().

Signed-off-by: Vincent Li <vincent.mc.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210719223022.66681-1-vincent.mc.li@gmail.com