package/mpd: use proper BR2_PACKAGE_MPD_VERSION_0_19 symbol

Commit 9244ad85 [1] adds a hidden choice between version 0.19 and 0.20. The
Makefile now uses a wrong symbol introduced by this commit. Fix by:
s/BR2_MPD_VERSION_0_19/BR2_PACKAGE_MPD_VERSION_0_19.

[1] https://git.buildroot.net/buildroot/commit/?id=9244ad854d30e8c942460646ef6f6194ee44942d



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>