The buildtar script might want to invoke a make, so tell the parent
make to pass the jobserver token pipe to the subcommand by prefixing
the command with a +.

This addresses the issue seen here:

  /bin/sh ../scripts/package/buildtar tar-pkg
  make[3]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

See https://www.gnu.org/software/make/manual/html_node/Job-Slots.html
for more information.
Signed-off-by: Trevor Bourget <tgb.kernel@gmail.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>