UAPI: Make uapi/linux/irqnr.h non-empty

uapi/linux/irqnr.h was emitted by the UAPI disintegration script as an empty
file because the parent linux/irqnr.h had no UAPI stuff in it, despite being
marked with "header-y".

Unfortunately, the patch program deletes the empty file when applying a kernel
patch.

It's not clear why this file is part of the UAPI at all.  Looking in:

	/usr/include/linux/irqnr.h

there's nothing there but a header reinclusion guard and a comment.

So just stick a comment in there as a placeholder.

Without this, if the kernel is fabricated from, say, a tarball and a patch, you
can get this error when building x86_64 or usermode Linux (and probably
others):

include/linux/irqnr.h:4:30: fatal error: uapi/linux/irqnr.h: No such file or directory
Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Reported-by: Alessandro Suardi <alessandro.suardi@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Randy Dunlap <rdunlap@xenotime.net>
cc: Alessandro Suardi <alessandro.suardi@gmail.com>

include/uapi/linux/irqnr.h

+/*
+ * There isn't anything here anymore, but the file must not be empty or patch
+ * will delete it.
+ */