sysutils/tmux: unbreak UTF8PROC on FreeBSD 14-CURRENT

...and set UTF8PROC_USES=localbase. On FreeBSD 13 and older,
the utf8proc version works without because we have USES=ncurses,
which pulls in the localbase equivalent through
ports/Mk/Uses/ncurses.mk. FreeBSD 14 builds use base ncurses instead,
so we need to make localbase explicit for utf8proc.

No PORTREVISION bump required because we would not have packages
to flush out.

Reported by: diizzy@
Approved by: portmgr@ (blanket)