selftests/bpf: Initialize tc in xdp_synproxy

xdp_synproxy/xdp fails in CI with:

Error: bpf_tc_hook_create: File exists

The XDP version of the test should not be calling bpf_tc_hook_create();
the reason it's happening anyway is that if we don't specify --tc on the
command line, tc variable remains uninitialized.

Fixes: 784d5dc0efc2 ("selftests/bpf: Add selftests for raw syncookie helpers in TC mode")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Reported-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230202235335.3403781-1-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>