devel/cligen: update to 6.4.0

ChangeLog: https://github.com/clicon/cligen/releases/tag/6.4.0

• Changed tv parameter of time2str() from struct to a pointer.
• Removed obsolete functions pt_name_get() / pt_name_set()

Corrected Bugs

• Output pipes released in 6.3 have several minor bugfixes
• Fixed: CLI show config | display exits over mountpoints with large YANGs
• Fixed: Escaping with backslash() in variable strings cannot be done for backslash itself

PR: 274216
Reported by: rcm@rcm.sh
Approved by: dave@dogwood.com (maintainer, MIA)