tools headers: Grab copy of linux/const.h, needed by linux/bits.h

So that can update the copy of linux/bits.h that now uses macros defined
in const.h and that are not available in older systems.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-c2qfcbl58hxyfb5u5xivp7is@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>