package/opencv: gpu module depends on !BR2_STATIC_LIBS

GPU support and related features (GL and CL) are detected and loaded at
runtime, so it requires libdl which is only available on
!BR2_STATIC_LIBS builds.

Because the OpenCV tests sets use these modules, they also requires
libdl, so they cannot be built when BR2_STATIC_LIBS is enable.

Fixes:
  http://autobuild.buildroot.net/results/570/57007a8d22d20b2fc5cd64154f5ec674b0842afa/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>