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

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

We'll use it to create tables for the 'protocol' argument to the
socket syscall when the 'family' arg is one of AF_INET or AF_INET6.

Add it to check_headers.sh so that when a new protocol gets added we get
a notification during the build process.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-2amnveu1ns4emjn70xuavpje@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>