update zlib to 1.2.5

"Version 1.2.5 fixes bugs in gzseek() and gzeof() that were present in version 1.2.4 (March 2010). All users are encouraged to upgrade immediately."
...

git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@7076 8ced0084-cf51-0410-be5f-012b33b47a6e

Externals/zlib/deflate.c

@@ -52,7 +52,7 @@
 #include "deflate.h"
 
 const char deflate_copyright[] =
-   " deflate 1.2.4 Copyright 1995-2010 Jean-loup Gailly and Mark Adler ";
+   " deflate 1.2.5 Copyright 1995-2010 Jean-loup Gailly and Mark Adler ";
 /*
   If you use the zlib library in a product, an acknowledgment is welcome
   in the documentation of your product. If for some reason you cannot