trace: fix recommendation given by error message in case dpkt is missing

Since we adveritse use of a system package in README, we should also
recommend the same here.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

scripts/trace.py

@@ -236,8 +236,7 @@ def needs_dpkt():
     try:
         import dpkt
     except ImportError:
-        raise Exception("""Cannot import dpkt. If you don't have it installed you can get it from
-             https://code.google.com/p/dpkt/downloads""")
+        raise Exception("""Cannot import dpkt. Please install 'python-dpkt' system package.""")
 
 def write_sample_to_pcap(sample, pcap_writer):
     ts = sample.time / 1e9