Expose zlib constants for backwards compat zlib-ng#747

Building a project that had an internal copy of minizip 1.x, but
preferring an external library, with minizip 4.x showed that the
compat layer was not exposing some constants from zlib that
minizip 1.x headers did.

mz_compat_shim.h.in

@@ -4,9 +4,14 @@
    This program is distributed under the terms of the same license as zlib.
    See the accompanying LICENSE file for the full text of the license.
 */
+
 #ifndef @MZ_COMPAT_FILE@
 #define @MZ_COMPAT_FILE@
 
 #include "mz_compat.h"
 
-#endif
+#ifndef ZLIB_H
+#include "zlib.h"
+#endif
+
+#endif