MIPS: Highmem: Fix build error

arch/mips/mm/highmem.c: In function 'kmap_init':
arch/mips/mm/highmem.c:130: error: 'init_mm' undeclared (first use in this function)
arch/mips/mm/highmem.c:130: error: (Each undeclared identifier is reported only once
arch/mips/mm/highmem.c:130: error: for each function it appears in.)
Signed-off-by: NYoichi Yuasa <yuasa@linux-mips.org>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/980/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/mm/highmem.c

 #include <linux/module.h>
 #include <linux/highmem.h>
+#include <linux/sched.h>
 #include <linux/smp.h>
 #include <asm/fixmap.h>
 #include <asm/tlbflush.h>