Add support for raw IP pcap files

Files created from Wireshark by using "File -> Strip Headers" and that
contain only the minimum amount of information to send RTP packets.
No headers to strip in this case so set offset to 0.

Documentation for DLT_RAW / raw IP / link layer 12 is available at:
https://github.com/the-tcpdump-group/libpcap/blob/master/pcap/dlt.h

src/prepare_pcap.c

@@ -148,6 +148,11 @@ size_t get_ethertype_offset(int link, const uint8_t* pktdata)
     if (link == DLT_EN10MB) {
         /* srcmac[6], dstmac[6], ethertype[2] */
         offset = 12;
+    /* Layer 3 IP packets / raw IP
+     * https://github.com/the-tcpdump-group/libpcap/blob/master/pcap/dlt.h#L111
+     */
+    } else if (link == DLT_RAW) {
+        return 0;
     } else if (link == DLT_LINUX_SLL) {
         /* http://www.tcpdump.org/linktypes/LINKTYPE_LINUX_SLL.html */
         /* pkttype[2], arphrd_type[2], lladdrlen[2], lladdr[8], ethertype[2] */