Configuration: only include shared_sources in dirinfo in shared config

Without this precaution, we end up having directory targets depend on
shlib object files for which there are no rules.

Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/7645)