bpf: make TC vlan bpf_helpers avail to selftests

The helper bpf_skb_vlan_push is needed by next patch, and the helper
bpf_skb_vlan_pop is added for completeness, regarding VLAN helpers.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>