selftests/bpf: Convert static to global in tc_redirect progs

Both IFINDEX_SRC and IFINDEX_DST are set from the userspace
and it won't work once bpf merges with bpf-next.

Fixes: 096eccde ("selftests/bpf: Rewrite test_tc_redirect.sh as prog_tests/tc_redirect.c")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210514170528.3750250-1-sdf@google.com