libstatgrab: correctly detect new signature for `sg_disk_io_stats()`

Enable `-Werror` to turn the warning into an error. I would have loved
using `-Werror=incompatible-pointer-types` but it is only supported with
very recent versions of gcc. Otherwise, it would just throw an error.

Also see #806 for an alternate solution. I think that #806 is better because relying on `-Werror` to only trigger an error on signature mismatch seems fragile. We could get other kind of warnings, like "comparison always true" when compilers will get smarter.