zlib: fix DEBUG build

The previous commit imported a little too much from upstream.  We need
to disable stdio.h when using U-Boot.

Reported-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

lib/zlib/zutil.h

@@ -91,7 +91,8 @@ extern const char * const z_errmsg[10]; /* indexed by 2-zlib_error */
 
 /* Diagnostic functions */
 #ifdef DEBUG
-#  include <stdio.h>
+/* Not valid for U-boot
+#  include <stdio.h> */
    extern int z_verbose;
    extern void z_error    OF((char *m));
 #  define Assert(cond,msg) {if(!(cond)) z_error(msg);}