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

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

So that we make sure we have recent enough defines for things
such as 'perf trace' system call argument beautifiers.

For instance, the 'clone' syscall argument 'flag' needs to use
CLONE_NEWCGROUP, and that is not available in RHEL7.

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: http://lkml.kernel.org/n/tip-81sln0ng4a2lcxrth14vcov4@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>