kbuild: rpm-pkg: fix jobserver unavailable warning

[ Upstream commit 606625be47bc87b6fab0af10cd57aaa675cb9e42 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>