Use https instead of ssh for submodule

- So it can be properly cloned in CI

[#135234395]

Signed-off-by: Anna Thornton <athornton@pivotal.io>

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "compile-extensions"]
 	path = compile-extensions
-	url = git@github.com:cloudfoundry/compile-extensions.git
+	url = https://github.com/cloudfoundry-incubator/compile-extensions.git