package/ffmpeg: fix build without pthreads

Fixes
http://autobuild.buildroot.org/results/4c7/4c77c15eaa2e44d0854cf24ab872e7b2ef83ebd6/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>