selftests/bpf: Fix test_lwt_seg6local.sh hangs

OpenBSD netcat (Debian patchlevel 1.195-2) does not seem to react to
SIGINT for whatever reason, causing prefix.pl to hang after
test_lwt_seg6local.sh exits due to netcat inheriting
test_lwt_seg6local.sh's file descriptors.

Fix by using SIGTERM instead.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200720101810.84299-1-iii@linux.ibm.com