Add SMPTE240M transfer characteristics flag.

(cherry picked from commit 4266191919c31a1921b497ec831d3b1c1fa28df8)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/avcodec.h

@@ -504,6 +504,7 @@ enum AVColorTransferCharacteristic{
     AVCOL_TRC_UNSPECIFIED=2,
     AVCOL_TRC_GAMMA22    =4, ///< also ITU-R BT470M / ITU-R BT1700 625 PAL & SECAM
     AVCOL_TRC_GAMMA28    =5, ///< also ITU-R BT470BG
+    AVCOL_TRC_SMPTE240M  =7,
     AVCOL_TRC_NB           , ///< Not part of ABI
 };