tools, include: Grab a copy of linux/erspan.h

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

Fixes: 933a741e3b82 ("selftests/bpf: bpf tunnel test.")
Reported-by: Yonghong Song <yhs@fb.com>
Signed-off-by: William Tu <u9012063@gmail.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>