libbase58: Use git URI for submodule to avoid failure on systems without HTTPS support

.gitmodules

@@ -6,4 +6,4 @@
 	url = git://git.ozlabs.org/~ccan/ccan
 [submodule "libbase58"]
 	path = libbase58
-	url = https://github.com/luke-jr/libbase58.git
+	url = git://github.com/luke-jr/libbase58.git