perf tools: Add the tools/ stringify copy to the MANIFEST

perf tools: Add the tools/ stringify copy to the MANIFEST

So that we don't end up using the kernel one when building out of tree,
via a detached tarball.

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>
Fixes: 737ef7d32cb4 ("tools include: Copy linux/stringify.h from the kernel")
Link: http://lkml.kernel.org/n/tip-t8yn1d7y0magk889ymc8jlai@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>