package/opencv3: use BR2_TOOLCHAIN_HAS_LIBATOMIC

It uses __atomic_fetch_add_4 so libatomic must be pulled in if
necessary. Fixes
http://autobuild.buildroot.net/results/652/6521223b426c3cb4dfefab27bc6021900db6910e/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>