nilfs-utils: needs threads support

nilfs-utils: needs threads support

nilfs-utils uses the sem_* family of functions from the realtime
extension. Technically this is not part of pthreads. But in uClibc
enabling threads enables also UCLIBC_HAS_REALTIME.

Fixes:
http://autobuild.buildroot.net/results/c5d/c5d93b6ab5b260fe5db46298b956a9ae765c828a/
http://autobuild.buildroot.net/results/9d6/9d6a248f9acbb58b6c2893de58978c6013332415/

Cc: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>