erlang: always use Buildroot's zlib

erlang: always use Buildroot's zlib

While Erlang includes a version of zlib, it's intended for Windows and
there's an expectation that non-Windows platforms provide it. It's also
not as regularly updated as the one in Buildroot. This change makes
Erlang always use a Buildroot-provided zlib.

Fixes this compile error:

 CC	/home/buildroot/autobuild/run/instance-0/output/build/erlang-21.0/erts/emulator/zlib/obj/x86_64-buildroot-linux-musl/opt/adler32.o
In file included from zlib/adler32.c:11:0:
zlib/zutil.h:172:39: error: "_LFS64_LARGEFILE" is not defined [-Werror=undef]
     (!defined(_LARGEFILE64_SOURCE) || _LFS64_LARGEFILE-0 == 0)
                                       ^~~~~~~~~~~~~~~~

See http://autobuild.buildroot.net/results/fc633f80c7c36a90e641487f5a888fbb767c2a54/.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>