Use https for librfxcodec reference

The git protocol is so limited that GitHub doesn't show it when selecting "Clone
of download". It may be blocked by strict firewalls, whereas https would work
almost certainly. Pushing code is not allowed over git protocol, but it's
possible over https with a tocken.

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "librfxcodec"]
 	path = librfxcodec
-	url = git://github.com/neutrinolabs/librfxcodec
+	url = https://github.com/neutrinolabs/librfxcodec.git