test/thash: fix build with clang 13

test/thash: fix build with clang 13

run_rss_calc() is used to compare the number of cycles spent computing a
hash value for different implementations.
clang 13 reports the hash variable as being unused, but run_rss_calc()
needs this variable as a placeholder for computing the hash value.

Bugzilla ID: 881
Fixes: 239fffe0402e ("test/thash: add performance tests for Toeplitz hash")
Reported-by: Liang Longfeng <longfengx.liang@intel.com>
Signed-off-by: Conor Walsh <conor.walsh@intel.com>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>