Use permanent links to library sources.

All zlib versions are stored in the fossils/ path of the website. Use
this path to avoid link rot.

Signed-off-by: Tu Dinh Ngoc <dinhngoc.tu@irit.fr>
Reviewed-by: Eduard-Florin Mihailescu <mihailescu.eduard@gmail.com>
Reviewed-by: Teodor Tiron <teotiron@gmail.com>
Reviewed-by: Razvan Deaconescu <razvand@unikraft.io>
Approved-by: Maria Sfiraiala <maria.sfiraiala@gmail.com>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #5

Author: dinhngtu

Makefile.uk

@@ -40,7 +40,7 @@ $(eval $(call addlib_s,libzlib,$(CONFIG_LIBZLIB)))
 # Original sources
 ################################################################################
 LIBZLIB_VERSION=1.2.13
-LIBZLIB_URL=http://www.zlib.net/zlib-$(LIBZLIB_VERSION).tar.gz
+LIBZLIB_URL=http://www.zlib.net/fossils/zlib-$(LIBZLIB_VERSION).tar.gz
 LIBZLIB_DIR=zlib-$(LIBZLIB_VERSION)
 
 $(eval $(call fetch,libzlib,$(LIBZLIB_URL),$(LIBZLIB_VERSION).tar.gz))