component/zlib: Fix zlib download

zlib source url has been moved. Please see issue
https://github.com/madler/zlib/issues/649 for reference.

component/zlib/buildout.cfg

@@ -5,5 +5,5 @@ parts =
 [zlib]
 recipe = slapos.recipe.cmmi
 shared = true
-url = https://www.zlib.net/zlib-1.2.12.tar.gz
+url = https://zlib.net/fossils/zlib-1.2.12.tar.gz
 md5sum = 5fc414a9726be31427b440b434d05f78