package/dvblast: needs threads

Fixes;
    http://autobuild.buildroot.org/results/c25/c25143fd1363d162de6733bc55a84289543f4da3/
    http://autobuild.buildroot.org/results/09d/09d9a962b20295d09d7777f873f7bb1c8130dbf1/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Julian Scheel <julian@jusst.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>