MIPS: PNX8550: Fix build error, broken by:

    commit 5a0e3ad6
    Author: Tejun Heo <tj@kernel.org>
    Date:   Wed Mar 24 17:04:11 2010 +0900

        include cleanup: Update gfp.h and slab.h includes to prepare for breaking implicit slab.h inclusion from percpu.h

Since a while the few headers included don't drag in <linux/kernel.h>
anymore, thus no more prototype of printk() resulting in:

  CC      arch/mips/nxp/pnx8550/common/reset.o
/home/ralf/src/linux/upstream-linus/arch/mips/nxp/pnx8550/common/reset.c: In function 'pnx8550_machine_restart':
/home/ralf/src/linux/upstream-linus/arch/mips/nxp/pnx8550/common/reset.c:31: error: implicit declaration of function 'printk'
/home/ralf/src/linux/upstream-linus/arch/mips/nxp/pnx8550/common/reset.c:33: error: 'NULL' undeclared (first use in this function)
/home/ralf/src/linux/upstream-linus/arch/mips/nxp/pnx8550/common/reset.c:33: error: (Each undeclared identifier is reported only once
/home/ralf/src/linux/upstream-linus/arch/mips/nxp/pnx8550/common/reset.c:33: error: for each function it appears in.)
make[3]: *** [arch/mips/nxp/pnx8550/common/reset.o] Error 1

Fixed by including <linux/kernel.h>

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