Fixup AVR builds. (#16875)

* Fixup AVR builds.

* Update platforms/avr/hardware_id.c

Co-authored-by: Ryan <fauxpark@gmail.com>

Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

platforms/avr/hardware_id.c

@@ -1,6 +1,12 @@
 // Copyright 2022 QMK
 // SPDX-License-Identifier: GPL-2.0-or-later
 
+// For some reason this bit is undocumented for some AVR parts and not defined in their avr-libc IO headers
+// See https://stackoverflow.com/questions/12350914/how-to-read-atmega-32-signature-row
+#ifndef SIGRD
+#    define SIGRD 5
+#endif // SIGRD
+
 #include <avr/boot.h>
 #include "hardware_id.h"