main/postmarketos-ondev: depend on ttf-dejavu

Proper fix for the monospace fonts seen when postmarketos-ondev is installed without also installing osk-sdl. This doesn't happen anymore with pmbootstrap install --ondev, because it will always pull in osk-sdl again for other reasons. However, since postmarketos-ondev is using this font, let's directly depend on it instead of having it pulled in as side effect of also having osk-sdl installed.

Fixes https://gitlab.com/postmarketOS/postmarketos-ondev/-/issues/50