Change zlib URL because the one from zlib.net requires a human-verification

VK-GL-CTS issue: 4895

This does not affect any test.

Change-Id: I7002062f46f957dd865d66387dde3e53d3b2c5f0
(cherry picked from commit b61f15f09adb6b7c9eefc7f7c44612c0c390abe5)
diff --git a/external/fetch_sources.py b/external/fetch_sources.py
index d00848c..835bc43 100644
--- a/external/fetch_sources.py
+++ b/external/fetch_sources.py
@@ -230,7 +230,7 @@
 
 PACKAGES = [
 	SourcePackage(
-		"http://zlib.net/fossils/zlib-1.2.13.tar.gz",
+		"https://github.com/madler/zlib/releases/download/v1.2.13/zlib-1.2.13.tar.gz",
 		"zlib-1.2.13.tar.gz",
 		"b3a24de97a8fdbc835b9833169501030b8977031bcb54b3b3ac13740f846ab30",
 		"zlib"),