package/dvblast: requires NPTL

package/dvblast: requires NPTL

dvblast uconditionally believes that any Linux system will have support
for clock nanosleep().

However, clock_nanosleep() is only availabe with NPTL.

Fixing dvblast is not really complicated, but not trivial either. So we
just make it depend on NPTL.

Fixes:
    http://autobuild.buildroot.org/?reason=dvblast-3.0
    http://autobuild.buildroot.org/results/dbe/dbe91dd6543b4b88fbf1e5b09d4997d2dc730747/
    http://autobuild.buildroot.org/results/32b/32bee5071f46b86de61db7f35a2cd04e270bc73c/
    [and many more...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>