Include zlib.h which is no longer implicitly included with libpng-1.5
Bug 369663

diff -ru tvtime-111b28cca42d.orig/src/pngoutput.c tvtime-111b28cca42d/src/pngoutput.c
--- tvtime-111b28cca42d.orig/src/pngoutput.c	2011-02-01 02:35:26.000000000 +0100
+++ tvtime-111b28cca42d/src/pngoutput.c	2011-06-02 13:36:55.965999463 +0200
@@ -18,6 +18,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <zlib.h>
 #include <png.h>
 #include "pngoutput.h"