[MIPS] IP27: Fix build errors if CONFIG_MAPPED_KERNEL=y

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/kernel/head.S b/arch/mips/kernel/head.S
index 3613645..492a0a8 100644
--- a/arch/mips/kernel/head.S
+++ b/arch/mips/kernel/head.S
@@ -22,6 +22,7 @@
 #include <asm/irqflags.h>
 #include <asm/regdef.h>
 #include <asm/page.h>
+#include <asm/pgtable-bits.h>
 #include <asm/mipsregs.h>
 #include <asm/stackframe.h>