nilfs-utils: need NPTL threads

nilfs-utils: need NPTL threads

nilfs-utils use clock_nanosleep(), which comes with NPTL threads. We
keep the dependency on threads, because sem_open() is really related
to thread support. The dependency on NPTL for clock_nanosleep() might
be lifted in the future, as it seems to be a bug in uClibc-ng.

Signed-off-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
[Thomas: update Config.in comment.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>