MIPS: MSP71xx: Fix build error.

After the recent cleanup of the register_*_smp_ops() functions msp71xx
wasn't fixed to include the now necessary header resulting in:

/home/ralf/src/linux/upstream-linus/arch/mips/pmc-sierra/msp71xx/msp_setup.c: In function ‘prom_init’:
/home/ralf/src/linux/upstream-linus/arch/mips/pmc-sierra/msp71xx/msp_setup.c:231:2: error: implicit declaration of function ‘register_vsmp_smp_ops’ [-Werror=implicit-function-declaration]
cc1: all warnings being treated as errors

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>