Fix install of CRAS ALSA plugin.

`install -D` only works as intended if the destination is the full filename (it only creates parent directories of the destination).
Since we are installing to a directory, it only creates its parents and not the directory itself...

This patch removes `-D`, and calls `mkdir -p` beforehand.
Merge pull request #391 from drinkcat/audio_fix_install

Fix install of CRAS ALSA plugin.