selftests/bpf: install with_tunnels.sh for test_flow_dissector.sh

test_flow_dissector.sh depends on both with_addr.sh and with_tunnels.sh
However, we install only with_addr.sh.

Add with_tunnels.sh to TEST_PROGS_EXTENDED to make sure it gets
installed as well.

Tested with: make TARGETS=bpf install INSTALL_PATH=$PWD/x

Fixes: ef4ab844

 ("selftests: bpf: install script with_addr.sh")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>