the log_file is defined in ftp.bro on line 54 as following:
...............
global log_file = open_log_file("ftp") &redef;
Yes, but that's part of an "export" that's inside "scope FTP". So you
need to use:
global FTP::log_file = open_log_file("ftp") &redef;
to redef it.
Vern