mmzone.h: remove dead prototype

get_zone_counts() was dropped from kernel tree, see:
http://www.mail-archive.com/mm-commits@vger.kernel.org/msg07313.html but
its prototype remains.
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -651,8 +651,6 @@ typedef struct pglist_data {
 #include <linux/memory_hotplug.h>
 
 extern struct mutex zonelists_mutex;
-void get_zone_counts(unsigned long *active, unsigned long *inactive,
-			unsigned long *free);
 void build_all_zonelists(void *data);
 void wakeup_kswapd(struct zone *zone, int order);
 int zone_watermark_ok(struct zone *z, int order, unsigned long mark,