message: videoio: fix ffmpeg standalone build
diff: #ifndef CV_UNUSED // Required for standalone compilation mode (OpenCV defines this in base.hpp)
  #define CV_UNUSED(name) (void)name
  #endif
  +#ifndef CV_Assert // Required for standalone compilation mode (OpenCV defines this in base.hpp)
  +#include <assert.h>
  +#define CV_Assert(expr) assert(expr)
  +#endif
  #ifdef __cplusplus
  extern "C" {
  