scripts/package: don't break if %{_smp_mflags} isn't set

Currently, if we do a 'make rpm-pkg' without the _smp_mflags rpm macro
defined, the build fails with:

  [snip]
  Executing(%build): /bin/bash -e /var/tmp/rpm-tmp.67959
  + umask 022
  + cd /home/jk/devel/kernel-snapshot/rpm/BUILD
  + cd kernel-2.6.26
  + make clean
  + make '%{_smp_mflags}'
  make[3]: *** No rule to make target `%{_smp_mflags}'.  Stop.
  error: Bad exit status from /var/tmp/rpm-tmp.67959 (%build)

This change uses the 'null if not set' reference to the _smp_mflags
macro instead.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>