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>

app/test/test_thash_perf.c

@@ -61,6 +61,9 @@ run_rss_calc(uint32_t *tuples[BATCH_SZ], enum test_rss_type type, int len,
 	}
 	end_tsc = rte_rdtsc_precise();
 
+	/* To avoid compiler warnings set hash to used. */
+	RTE_SET_USED(hash);
+
 	return end_tsc - start_tsc;
 }