package/tmux: add utf8proc optional dependency

utf8proc is an optional dependency since version 2.3 and
https://github.com/tmux/tmux/commit/6c94774b70f72952c4c512e4aa59a207ca1c34f2



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>