zlib.c: avoid build conflicts for cradle board

Commit dce3d797 updated the zlib code to v0.95; this caused
conflicts when building for the "cradle" board, because the (pretty
generic) preprocessor variable "OFF" was used in multiple files.
Make sure to avoid further conflicts by #undef'ing it in zlib.c
before redefining it.

Signed-off-by: Wolfgang Denk <wd@denx.de>
cc: Giuseppe Condorelli <giuseppe.condorelli@st.com>
cc: Angelo Castello <angelo.castello@st.com>
cc: Alessandro Rubini <rubini-list@gnudd.com>