tracing04: add some notes on how to build the example

In order to build the xdp_sample_pkts_user program, the libpcap-dev and the
libc-dev libraries should be installed. Note this in the assignment.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>

Author: aspsk

tracing04-xdp-tcpdump/README.org

@@ -100,7 +100,9 @@ PING fc00:dead:cafe:1::1(fc00:dead:cafe:1::1) 56 data bytes
 
 ** Assignment 2: The PCAP dump file
 
-Load the eBPF kernel packets dump program and store the packets to the dump file:
+Build the =xdp_sample_pkts_user= dump program; to do so you might have to
+install the =libpcap-dev= and the 32 bit libc dev packages.  Load the eBPF
+kernel packets dump program and store the packets to the dump file:
 
 #+begin_example sh
 $ sudo ./xdp_sample_pkts_user -d veth-basic02 -F