Add the common directory back to kernel headers.

This is a port of build/ change 6a2f28514.

Change-Id: I9128abf61a56e282f335f540fcf064dd52706b2b
diff --git a/cc/x86_device.go b/cc/x86_device.go
index d0bd8a0..e47c6fb 100644
--- a/cc/x86_device.go
+++ b/cc/x86_device.go
@@ -171,6 +171,7 @@
 		"-isystem ${LibcRoot}/arch-x86/include",
 		"-isystem ${LibcRoot}/include",
 		"-isystem ${LibcRoot}/kernel/uapi",
+		"-isystem ${LibcRoot}/kernel/common",
 		"-isystem ${LibcRoot}/kernel/uapi/asm-x86",
 	}, " "))