Fix "Missing configuration" message in dist for WT_STORAGE_SOURCE

When run dist scripts, we see this:

 $ python api_config.py 

Missing configuration for WT_STORAGE_SOURCE.customize_file_system in ../src/include/wiredtiger.in

It doesn't cause an error when running the scripts, but should be addressed.

Even though `WT_STORAGE_SOURCE` is not called directly (and is currently ifdef'ed out for DOXYGEN as part of "unsupported" APIs), its APIs could probably be managed via `api_data.py`.