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/gzclose.c

@@ -0,0 +1,25 @@
+/* gzclose.c -- zlib gzclose() function
+ * Copyright (C) 2004, 2010 Mark Adler
+ * For conditions of distribution and use, see copyright notice in zlib.h
+ */
+
+#include "gzguts.h"
+
+/* gzclose() is in a separate file so that it is linked in only if it is used.
+   That way the other gzclose functions can be used instead to avoid linking in
+   unneeded compression or decompression routines. */
+int ZEXPORT gzclose(file)
+    gzFile file;
+{
+#ifndef NO_GZCOMPRESS
+    gz_statep state;
+
+    if (file == NULL)
+        return Z_STREAM_ERROR;
+    state = (gz_statep)file;
+
+    return state->mode == GZ_READ ? gzclose_r(file) : gzclose_w(file);
+#else
+    return gzclose_r(file);
+#endif
+}