msp71xx_defconfig can not be built at the in v4.14-rc1

arch/mips/pmcs-msp71xx/msp_smp.c:72:2: error: implicit declaration of function 'set_vi_handler' [-Werror=implicit-function-declaration]

I don't know what caused the regression, but including the right
header is the obvious fix.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17309/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>