subr_unit.c: another attempt to fix the build

Reported by:	cy
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/subr_unit.c

@@ -340,6 +340,10 @@ free_iter_unr(void *handle)
 }
 
 #if defined(DIAGNOSTIC) || !defined(_KERNEL)
+#ifndef __diagused
+#define	__diagused
+#endif
+
 /*
  * Consistency check function.
  *