app/testpmd: fix build with clang 15

This variable is used to create some artificial load.

Fixes: 3c156061b9 ("app/testpmd: add noisy neighbour forwarding mode")
Cc: stable@dpdk.org

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Tyler Retzlaff <roretzla@linux.microsoft.com>

app/test-pmd/noisy_vnf.c

@@ -57,8 +57,8 @@ do_write(char *vnf_mem)
 static inline void
 do_read(char *vnf_mem)
 {
+	uint64_t r __rte_unused;
 	uint64_t i = rte_rand();
-	uint64_t r;
 
 	r = vnf_mem[i % ((noisy_lkup_mem_sz * 1024 * 1024) /
 			RTE_CACHE_LINE_SIZE)];