Fix for compatibility with libpng 1.4+

https://github.com/x3ro/VisualBoyAdvance/pull/1/commits/1292c3b70337cccddf52e8d5de23bd785687d92a
--- src/Util.cpp.orig	2004-05-20 12:42:37.000000000 -0500
+++ src/Util.cpp	2018-10-07 22:30:32.000000000 -0500
@@ -79,7 +79,7 @@
     return false;
   }
 
-  if(setjmp(png_ptr->jmpbuf)) {
+  if(setjmp(png_jmpbuf(png_ptr))) {
     png_destroy_write_struct(&png_ptr,NULL);
     fclose(fp);
     return false;