can-utils: add dependency on NPTL

can-utils: add dependency on NPTL

canfdtest.c assumes that clock_nanosleep() is always available. But it
is only available with NPTL. Since it unconditionally uses it, we add
dependency on NPTL.

Fixes:
http://autobuild.buildroot.net/results/fd97aa57747a546c799caf094aafe0d1d1225863
http://autobuild.buildroot.net/results/1313f91df134035159e677c6b4a63838c75d074c
http://autobuild.buildroot.net/results/a1ac495c5e38e5d0777c431fc6a2e4b5bff7c2ad
http://autobuild.buildroot.net/results/978bee40049896bd6e093fa31c5eb7673e05a6b8
http://autobuild.buildroot.net/results/c692e96de1b4fdadf720ead8249a44c21fe945f1

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>