MIPS: Make a needlessly global symbol static in arch/mips/kernel/smp.c

The variable cpu_callin_map is needlessly defined global, so let's
make it static now.

Build-tested using malta_defconfig.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>