Fix some non-ANSI function declaration

This pull request adds `void` in case no function parameters are declared.

This was originally submitted to Bryan upstream as a part of PR #5 , and it's broken out into its own pull request here for further review. 

 @dalgaaf originally submitted this to Ceph as part of https://github.com/ceph/libs3/pull/2 , reviewed/merged by @liewegas. Sage, Danny: do we need this in? Bryan was hoping for a bit more explanation since this is an older convention.