move extern repos to https addresses

The extern repos are currently cloned with SSH. This means that
attempting to clone them on a system that doesn't have an SSH key which
GitHub accepts fails. Change them to HTTPS as they're read only anyway.

.gitmodules

@@ -1,9 +1,9 @@
 [submodule "extern/drgn"]
-	path = extern/drgn
-  url = git@github.com:JakeHillion/drgn.git
+  path = extern/drgn
+  url = https://github.com/JakeHillion/drgn.git
 [submodule "extern/folly"]
-	path = extern/folly
-	url = git@github.com:jgkamat/folly.git
+  path = extern/folly
+  url = https://github.com/jgkamat/folly.git
 [submodule "extern/rocksdb"]
-	path = extern/rocksdb
-	url = git@github.com:facebook/rocksdb.git
+  path = extern/rocksdb
+  url = https://github.com/facebook/rocksdb.git