$NetBSD$

Fix build with png-1.5.

--- converter/other/pngtxt.h.orig	2006-08-19 04:12:28.000000000 +0100
+++ converter/other/pngtxt.h	2011-01-14 21:39:26.000000000 +0000
@@ -5,7 +5,8 @@
 #include <png.h>
 
 void 
-pnmpng_read_text (png_info * const info_ptr, 
+pnmpng_read_text (png_struct * png_ptr,
+                  png_info * const info_ptr, 
                   FILE *     const tfp, 
                   bool const ztxt,
                   bool const verbose);
