portaudio: needs threads

Fixes:
http://autobuild.buildroot.org/results/ba2/ba2d3832cb6838b2126e0f7db223305eab579d59



[Peter: move above so sub options gets properly indented]
Signed-off-by: Gilles Talis <gilles.talis@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>