main/postmarketos-ui-fbkeyboard: fix dejavu font path

Alpine changed location and package name of ttf-dejavu fonts, adjust to new location.

Without this, fbkeyboard service crashes with error:

unable to load font file: No such file or directory

Default font path is hardcoded in https://github.com/bakonyiferenc/fbkeyboard/blob/master/fbkeyboard.c#L37 to /usr/share/fonts/ttf-dejavu/DejaVuSans.ttf. Luckily there is a CLI switch to change default, use it.

Fixes #1764 (closed)