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>