MISC: Dockerfile: add zstd for make distcheck in the CI

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

misc/Dockerfile

@@ -3,7 +3,7 @@ FROM gcc:latest
 RUN apt-get update && apt-get install -y \
   libgcrypt20-dev libsndfile1-dev libmpg123-dev libzita-resampler-dev \
   libfftw3-dev libavcodec-dev libavformat-dev autoconf-archive clang \
-  ffmpeg gettext
+  ffmpeg gettext zstd
 
 ADD . /audiowmark
 WORKDIR /audiowmark