message: fix linker error when trying CPU_BASELINE=AVX
diff: #include <opencv2/dnn.hpp>
  #include <opencv2/dnn/shape_utils.hpp>
  +#define CV_CPU_OPTIMIZATION_DECLARATIONS_ONLY
  // dispatched AVX/AVX2 optimizations
  #include "layers/layers_common.simd.hpp"
  #include "layers/layers_common.simd_declarations.hpp"
  +#undef CV_CPU_OPTIMIZATION_DECLARATIONS_ONLY
  namespace cv
  {
  