net/xorp: Fix dependency on libpcap (use ports version)

The specification on libpcap came on the v1.6 => v.1.8.5 transition
while the port was unmaintained. Assume the change originator intended
for the ports libpcap to be used, but didn't realize it needed an
explicit version number to avoid the base library from satisfying the
requirement. While here, remove an obsolete CONFLICTS definition.

PR: 211578