tiff: print log in case of unknown / unsupported tag.

Helps debugging.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/tiff.c

@@ -478,6 +478,8 @@ static int tiff_decode_tag(TiffContext *s, const uint8_t *start, const uint8_t *
         if(s->compr == TIFF_G4)
             s->fax_opts = value;
         break;
+    default:
+        av_log(s->avctx, AV_LOG_DEBUG, "Unknown or unsupported tag %d/0X%0X\n", tag, tag);
     }
     return 0;
 }