Fix wrong usage of gzwrite

https://github.com/x3ro/VisualBoyAdvance/pull/1/commits/8b5249d58edded5c8e4b2439eef7b46fabd59a36
--- src/Util.cpp.orig	2004-05-20 12:42:37.000000000 -0500
+++ src/Util.cpp	2018-10-07 22:34:37.000000000 -0500
@@ -984,7 +984,7 @@
 
 gzFile utilGzOpen(const char *file, const char *mode)
 {
-  utilGzWriteFunc = (int (*)(void *,void * const, unsigned int))gzwrite;
+  utilGzWriteFunc = (int (*)(gzFile,void * const, unsigned int))gzwrite;
   utilGzReadFunc = gzread;
   utilGzCloseFunc = gzclose;