readme: Add appkey.c as dep
readme: Add appkey.c as dep

Might as well be explicit.

Signed-off-by: Carl Helmertz <d78a787955449ad50f586e5155db5f9260d297a8@gmail.com>

catharsis/spotifile,chelmertz/spotifile,chelmertz/spotifile,catharsis/spotifile,raoulh/spotifile,raoulh/spotifile,catharsis/spotifile,chelmertz/spotifile,raoulh/spotifile