[werror] Fix mismatched tags (struct vs. class) inconsistence

Signed-off-by: Thorsten Behrens <tbehrens@suse.com>
I thought we'd done a switcheroo on this header before, but searching now i can only find 12238d0906ad0ba6306b00594dffba0191728200.

I think we want to stick with struct, though, since this is a C interface, not C++!