package/opencv: add openmp support

Note that this situation is not ideal because it only add openmp support
to the Buildroot toolchain, not the external ones; but a couple of
packages already are in this situation.

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