Reintroduce the JDK submodules

This partially reverts commit 24ec76b6ebb0c7496b2eb0a1b1738f8c28f4398e.

Otherwise, the Build.sh fails to build Fiji, emitting the error:

No JDK found; cloning it
No submodule mapping found in .gitmodules for path 'java/linux'
Could not clone JDK