tools/target_dec_fuzzer: enable mjpeg for tiff or tdsc

This is needed for fuzzing tiff/tdsc and should increase coverage

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/allcodecs.c

@@ -812,6 +812,7 @@ extern AVCodec ff_vp9_qsv_encoder;
 // The iterate API is not usable with ossfuzz due to the excessive size of binaries created
 #if CONFIG_OSSFUZZ
 AVCodec * codec_list[] = {
+    NULL,
     NULL,
     NULL
 };