bpf: make helper function protos static

They are only used here, so there's no reason they should not be static.
Only the vlan push/pop protos are used in the test_bpf suite.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>