build-aux: Use GitHub repo for LuaJIT

Curl reports that the luajit.org git repo is using a self-signed cert.
This is causing git operations to fail. Let's just use the GitHub
mirror.

build-aux/modules/40-luajit.json

@@ -10,7 +10,7 @@
     "sources": [
         {
             "type": "git",
-            "url": "https://luajit.org/git/luajit-2.0.git",
+            "url": "https://github.com/LuaJIT/LuaJIT.git",
             "commit": "a4f56a459a588ae768801074b46ba0adcfb49eb1",
             "disable-shallow-clone": true
         },