fix (iperf_api): fix compiler warning while printing an uint64_t (#969)

Fix the following warning:
../../src/iperf_api.c: In function ‘iperf_on_connect’:
../../src/iperf_api.c:791:58: warning: format ‘%llu’ expects argument
of type ‘long long unsigned int’, but argument 3 has type ‘uint64_t’
{aka ‘long unsigned int’} [-Wformat=]

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

src/iperf_api.c

@@ -773,7 +773,7 @@ iperf_on_connect(struct iperf_test *test)
             }
         }
         if (test->settings->rate)
-            iperf_printf(test, "      Target Bitrate: %llu\n", test->settings->rate);
+            iperf_printf(test, "      Target Bitrate: %"PRIu64"\n", test->settings->rate);
     }
 }