subr_unit.c: make userspace tests buildable

by defining a placeholder for UNR_NO_MTX

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

sys/kern/subr_unit.c

@@ -143,6 +143,8 @@ _Malloc(size_t foo, int line)
 
 struct unrhdr;
 
+#define	UNR_NO_MTX	((void *)(uintptr_t)-1)
+
 struct mtx {
 	int	state;
 } unitmtx;