mxfenc: fix ignored drop flag in binary timecode representation.

libavformat/mxfenc.c

@@ -1539,7 +1539,7 @@ static const uint8_t system_metadata_package_set_key[] = { 0x06,0x0E,0x2B,0x34,0
 static uint32_t ff_framenum_to_12m_time_code(unsigned frame, int drop, int fps)
 {
     return (0                                    << 31) | // color frame flag
-           (0                                    << 30) | // drop  frame flag
+           (drop                                 << 30) | // drop  frame flag
            ( ((frame % fps) / 10)                << 28) | // tens  of frames
            ( ((frame % fps) % 10)                << 24) | // units of frames
            (0                                    << 23) | // field phase (NTSC), b0 (PAL)