include: sys/arc.h: shim out arc_referenced()

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12844

include/sys/arc.h

@@ -295,6 +295,8 @@ void arc_buf_freeze(arc_buf_t *buf);
 void arc_buf_thaw(arc_buf_t *buf);
 #ifdef ZFS_DEBUG
 int arc_referenced(arc_buf_t *buf);
+#else
+#define	arc_referenced(buf) ((void) sizeof (buf), 0)
 #endif
 
 int arc_read(zio_t *pio, spa_t *spa, const blkptr_t *bp,