stella: fix build without threads support

stella: fix build without threads support

Fix build error when building using toolchain without threads support:

src/common/tv_filters/AtariNTSC.hxx:172:16: error: 'thread' is not a member of 'std'
     unique_ptr<std::thread[]> myThreads;
                     ^~~

Since version 5.0.2, Stella needs a toolchain with threads support.

Fixes:
http://autobuild.buildroot.net/results/bd30388ee24294158d0a373764408c8c846853d4
http://autobuild.buildroot.net/results/ad1571cecfc697650b436c147b5f3a1b4326091d
http://autobuild.buildroot.net/results/10b73362358f3af45534a0cd096672dd1460a7d0

Signed-off-by: Sergio Prado <sergio.prado@e-labworks.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>