opencv: since 2.4.8 bump, requires NPTL

The core module of OpenCV is now using the pthread_spin_*() API, which
is only available in the 'linuxthreads' and 'NPTL' thread
implementations, but not in the 'linuxthreads.old'
implementation. Therefore, this commit adds a dependency on OpenCV to
NPTL threads.

Fixes:

  http://autobuild.buildroot.org/results/75d/75db3827ec4be8a3805508a22b8b6a4d1a3b9ce3/



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