kbuild: rpm-pkg: fix jobserver unavailable warning

If "make rpm-pkg" or "make binrpm-pkg" is run with -j[jobs] option,
the following warning message is displayed.

  warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

Follow the suggestion.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>