ftpd(8): do not refer to now unused libxo(3)

In 2018, the commit r328100 (0fdf7fa846b1a1b1679e86812a1b08b8cb623604)
removed libxo(3) support from ls(1), so ftpd has no reasons to link
with libxo since then.

(cherry picked from commit 796d48ec416863ab2826a1205365710635244023)