Drop unused zlib header.

--- apng2gif-1.8-src/apng2gif.cpp
+++ apng2gif-1.8-src/apng2gif.cpp
@@ -33,7 +33,6 @@
 #include <string.h>
 #include <vector>
 #include "png.h"     /* original (unpatched) libpng is ok */
-#include "zlib.h"
 
 #define notabc(c) ((c) < 65 || (c) > 122 || ((c) > 90 && (c) < 97))