scripts/bpf: Fix xdp_md forward declaration typo

commit e0b68fb186b251374adbd870f99b1ecea236e770 upstream.

Fix typo in struct xpd_md, generated from bpf_helpers_doc.py, which is
causing compilation warnings for programs using bpf_helpers.h

Fixes: 7a387bed47f7 ("scripts/bpf: teach bpf_helpers_doc.py to dump BPF helper definitions")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191010042534.290562-1-andriin@fb.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>