alsa-plugins: fix libavtp vs. avtp packageconfig

Fix PACKAGECONFIG to refer to libavtp instead of avtp as this is what
the project and everything is really called everywhere.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 8824d91f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>