mpd: Add build dependency on samba

Version 0.19 [1], adds the smbclient plugins for storage and input so
needs samba when building with the --enable-samba option.

[1] http://git.musicpd.org/cgit/master/mpd.git/plain/NEWS?h=v0.19



Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>