This change is based on recent discussions of the JFFS2 code relying on 
constants defined in the private zlib header zutil.h

The result of this discussion is to change the use of the constant into 
a 'Magic Number' (according to Mark Adler this number is part of the standard 
and will never change - defined in RFC 1950)

Signed-off-by: Daniel Hazelton <dhazelton@enter.net>