package/bullet: extras needs NPTL

Fixes:
 - http://autobuild.buildroot.org/results/9c6325e6cc86116d1e8c940e0fcfe94243faa6de



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>