Add the common directory back to kernel headers.

This is a port of build/ change 6a2f28514.

Change-Id: I9128abf61a56e282f335f540fcf064dd52706b2b
diff --git a/cc/mips_device.go b/cc/mips_device.go
index 86e7dee..b2b2ccf 100644
--- a/cc/mips_device.go
+++ b/cc/mips_device.go
@@ -140,6 +140,7 @@
 		"-isystem ${LibcRoot}/arch-mips/include",
 		"-isystem ${LibcRoot}/include",
 		"-isystem ${LibcRoot}/kernel/uapi",
+		"-isystem ${LibcRoot}/kernel/common",
 		"-isystem ${LibcRoot}/kernel/uapi/asm-mips",
 	}, " "))