opencv: disable OpenMP support

Since there is currently no easy way to know if the current Buildroot
configuration has a toolchain supporting OpenMP (especially for external
toolchains), then just disable it.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>