The mips_mt_* symbols are only built and exported if
CONFIG_MIPS_MT is enabled.

Fixes the following build problem when CONFIG_SMP is enabled
but CONFIG_MIPS_MT is not.

arch/mips/built-in.o: In function `cmp_prepare_cpus':
arch/mips/kernel/smp-cmp.c:197:
undefined reference to `mips_mt_set_cpuoptions'
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5860/