Fix librdkafka Ruby extension file ext in whitelist_file
Fix librdkafka Ruby extension file ext in whitelist_file

In macOS => .dylib
Others => .so

Signed-off-by: Hiroshi Hatake <54b742842f0d940c9715551f77be1b560ea6cc67@gmail.com>

treasure-data/omnibus-td-agent,treasure-data/omnibus-td-agent,treasure-data/omnibus-td-agent