Bring the erspan uapi header file so BPF tunnel helpers can use it.

Fixes: 933a741e ("selftests/bpf: bpf tunnel test.")
Reported-by: NYonghong Song <yhs@fb.com>
Signed-off-by: NWilliam Tu <u9012063@gmail.com>
Acked-by: NYonghong Song <yhs@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>