Fix Deprecated DPDK Function in Speed Tester NF (#75)

The `rte_ctrlmbuf_data` function was removed in DPDK 18.05. This caused errors when attempting to compile the Speed Tester NF with pcap support enabled. Replaces the function with the `rte_pktmbuf_mtod` macro.

Commit log:

* Fixes deprecated rte function

Author: dennisafa

examples/speed_tester/speed_tester.c

@@ -398,7 +398,7 @@ nf_setup(struct onvm_nf_info *nf_info) {
                         pkt->data_len = header.caplen;
 
                         /* Copy the packet into the rte_mbuf data section */
-                        rte_memcpy(rte_ctrlmbuf_data(pkt), packet, header.caplen);
+                        rte_memcpy(rte_pktmbuf_mtod(pkt, char*), packet, header.caplen);
 
                         pmeta = onvm_get_pkt_meta(pkt);
                         pmeta->destination = destination;