call the less fake kunmap

Reference: /n/atom/patch/applied/pikunmap
Date: Thu Jan  7 00:40:20 CET 2016
Signed-off-by: quanstro@quanstro.net

--- /sys/src/9/bcm/dat.h	Thu Jan  7 00:39:31 2016
+++ /sys/src/9/bcm/dat.h	Thu Jan  7 00:39:33 2016
@@ -230,7 +230,7 @@
 typedef void		KMap;
 #define	VA(k)		((uintptr)(k))
 #define	kmap(p)		(KMap*)((p)->pa|kseg0)
-#define	kunmap(k)
+extern void kunmap(KMap*);
 
 struct
 {