build: Fix dependency on gen-libunistring-pot
build: Fix dependency on gen-libunistring-pot

Make it order-only so not to reproduce libunistring.pot every time
gen-libunistring-pot is built.

GNOME/gnome-characters,GNOME/gnome-characters,GNOME/gnome-characters,GNOME/gnome-characters,GNOME/gnome-characters