tools include uapi: Grab a copy of linux/vhost.h

tools include uapi: Grab a copy of linux/vhost.h

We will use it to generate tables for beautifying ioctl's 'cmd' arg.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-nxwpq34hu6te1m2ra5m7o8n9@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>