kbuild: mergeconfig: fix "jobserver unavailable" warning

If "make kvmconfig" is run with "-j" option, a warning message,
"jobserver unavailable: using -j1.  Add `+' to parent make rule.",
is displayed.

  $ make -s defconfig
  *** Default configuration is based on 'x86_64_defconfig'
  #
  # configuration written to .config
  #
  $ make -j8 kvmconfig
  Using ./.config as base
  Merging ./arch/x86/configs/kvm_guest.config
    [ snip ]
  #
  # merged configuration written to ./.config (needs make)
  #
  make[2]: warning: jobserver unavailable: using -j1.  Add `+' to
  parent make rule.
  scripts/kconfig/conf --oldconfig Kconfig
    [ snip ]
  #
  # configuration written to .config
  #

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>