ovs-tcpdump: Fix an undefined variable

Run ovs-tcpdump without --span, and it throws the following
exception. Define mirror_select_all to avoid the error.

Traceback (most recent call last):
  File "/usr/local/bin/ovs-tcpdump", line 488, in <module>
    main()
  File "/usr/local/bin/ovs-tcpdump", line 454, in main
    mirror_select_all)
UnboundLocalError: local variable 'mirror_select_all' referenced before assignment

Fixes: 0475db7 ("ovs-tcpdump: Add --span to mirror all ports on bridge.")
Acked-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

utilities/ovs-tcpdump.in

@@ -377,6 +377,7 @@ def main():
 
     skip_next = False
     mirror_interface = None
+    mirror_select_all = False
     dump_cmd = 'tcpdump'
 
     for cur, nxt in argv_tuples(sys.argv[1:]):