Update zlib-ng version to 2.0.3 (#90)

Context: https://github.com/zlib-ng/zlib-ng/releases/tag/2.0.3

Changes since 2.0.2:

 * Include porting guide in release packages
 * Documentation improvements
 * Added Windows ARM binaries in release packages
 * Fix crash on ARMv7
 * Fix building on FreeBSD
 * Fix building with musl on aarch64
 * Fix ARM float-abi detection
 * Fix cmake detection of risc-v architectures
 * Minor buildsystem fixes
 * Improve zlib-compat build
 * CI/Test improvements

Author: grendello