tools: zlib: switch to git source

tools: zlib: switch to git source

Switch to the git repository source so that we can use zstd
compression algorithm to create smaller package tarball. This
patch also corrected the license file name[1].

[1] https://github.com/madler/zlib/commit/352cb28d12baf02863ff5d4d96be0587ced419a1

Suggested-by: Christian Marangi <[email protected]>
Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/17880
Signed-off-by: Robert Marko <[email protected]>