text2pcap: Set encapsulation to WIRESHARK_UPPER_PDU with -P

The documentation, both man page and help, claims that text2pcap
automatically sets the encapsulation to WIRESHARK_UPPER_PDU if
-P is given. Make the behavior match the documentation.

text2pcap.c

@@ -480,6 +480,7 @@ parse_options(int argc, char *argv[], text_import_info_t * const info, wtap_dump
 
         case 'P':
             hdr_export_pdu = TRUE;
+            wtap_encap_type = WTAP_ENCAP_WIRESHARK_UPPER_PDU;
             info->payload = ws_optarg;
             break;