migration: Remove unnecessary zlib include from qemu-file.h

Commit 70eb5fde05 removed zlib from the libmigration dependencies but missed removing the zlib include from the header file. zlib doesn't appear to be used anywhere in qemu-file.h. This is fine in most environments where zlib is nearly always in the default include path but breaks the build when it isn't (NixOS for example). Fixes: 70eb5fde05 ("migration: remove unnecessary zlib dependency") Signed-off-by: Chad Jablonski <chad@jablonski.xyz> Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>