Add kbd-1.15.

This also requires a patch to linux-headers; while we are at it, make
linux-headers PREFIX agnostic.

sysa/kbd-1.15/patches/fcntl.patch

@@ -0,0 +1,16 @@
+SPDX-FileCopyrightText: 2021 fosslinux <fosslinux@aussies.space>
+
+SPDX-License-Identifier: GPL-2.0-or-later
+
+Add missing fcntl header.
+
+--- src/kbdrate.c	2021-07-22 22:35:53.671970814 +1000
++++ src/kbdrate.c	2021-07-22 22:36:10.038096797 +1000
+@@ -94,6 +94,7 @@
+ };
+ 
+ #include <signal.h>
++#include <fcntl.h>
+ 
+ #include "nls.h"
+ #include "version.h"