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

Same fix as in 13797b77 is needed for the "new"
line invoking _smp_mflags for modules_install. Without the fix, `make
binrpm-pkg` fails with:

+ make '%{_smp_mflags}' KBUILD_SRC= modules_install
make[2]: warning: jobserver unavailable: using -j1.  Add `+' to parent make rule.
make[2]: *** No rule to make target `%{_smp_mflags}'.  Stop.
error: Bad exit status from /var/tmp/rpm-tmp.8S9B9e (%install)

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>