samples: aio comparators: Use expected pointer type in printf

The data structure member being used  is character array,
dereferencig this array gives **char instead of the expected
*char type.

This issue was reported by Coverity

Coverity-CID: 152030
Coverity-CID: 152033

Change-Id: Ied67e4b2d47017e6ad5e40b9b6fca1b496c483ed
Signed-off-by: Sergio Rodriguez <sergio.sf.rodriguez@intel.com>