tiffenc: Add forgotten avclass to context.

(cherry picked from commit 43c481e569)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/tiffenc.c

@@ -42,6 +42,7 @@ static const uint8_t type_sizes2[6] = {
 };
 
 typedef struct TiffEncoderContext {
+    AVClass *avclass;
     AVCodecContext *avctx;
     AVFrame picture;