Add zstd dev package for building lld.

Author: khuey

src/ci/docker/host-x86_64/x86_64-gnu/Dockerfile

@@ -18,6 +18,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   xz-utils \
   mingw-w64 \
   zlib1g-dev \
+  libzstd-dev \
   && rm -rf /var/lib/apt/lists/*
 
 COPY scripts/sccache.sh /scripts/