video-converter: Fix compiler warnings

video-converter: Fix compiler warnings

video-converter.c:3073:48: error: implicit conversion from enumeration type 'GstFormat' to different enumeration type 'GstVideoFormat'
      [-Werror,-Wenum-conversion]
        gst_video_scaler_horizontal (h_scaler, format,
        ~~~~~~~~~~~~~~~~~~~~~~~~~~~            ^~~~~~
video-converter.c:3081:44: error: implicit conversion from enumeration type 'GstFormat' to different enumeration type 'GstVideoFormat'
      [-Werror,-Wenum-conversion]
      gst_video_scaler_vertical (v_scaler, format, lines, d, i, out_w);
      ~~~~~~~~~~~~~~~~~~~~~~~~~            ^~~~~~
video-converter.c:3137:24: error: implicit conversion from enumeration type 'const GstVideoFormat' to different enumeration type 'GstFormat'
      [-Werror,-Wenum-conversion]
    convert->fformat = GST_VIDEO_INFO_FORMAT (in_info);
                     ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../gst-libs/gst/video/video-info.h:125:43: note: expanded from macro 'GST_VIDEO_INFO_FORMAT'
                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../../gst-libs/gst/video/video-format.h:361:59: note: expanded from macro 'GST_VIDEO_FORMAT_INFO_FORMAT'
                                                  ~~~~~~~~^~~~~~
video-converter.c:3157:24: error: implicit conversion from enumeration type 'GstVideoFormat' to different enumeration type 'GstFormat'
      [-Werror,-Wenum-conversion]
    convert->fformat = GST_VIDEO_FORMAT_GRAY8;