ipipe: Add missing include for ipipe_root_only

Breaks in non-debug builds otherwise, e.g.
https://travis-ci.com/xenomai-ci/xenomai/jobs/212725223



Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

include/linux/kernel.h

@@ -15,6 +15,7 @@
 #include <linux/printk.h>
 #include <linux/build_bug.h>
 #include <asm/byteorder.h>
+#include <asm-generic/ipipe.h>
 #include <asm/div64.h>
 #include <uapi/linux/kernel.h>
 #include <asm/div64.h>