MIPS: Highmem: Fix build error if CONFIG_DEBUG_HIGHMEM is disabled

  CC      arch/mips/mm/highmem.o
/home/ralf/src/linux/linux-mips/arch/mips/mm/highmem.c: In function ‘__kunmap_atomic’:
/home/ralf/src/linux/linux-mips/arch/mips/mm/highmem.c:70:6: error: variable ‘type’ set but not used [-Werror=unused-but-set-variable]
cc1: all warnings being treated as errors

This warning exists in gcc 4.6.0 and newer.  Kernels 2.6.40 and newer use
-Wunused-but-set-variable to suppress it.

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