explicitly add llvm-9-dev in dockerfile

apparently llvm-8-tools already had llvm-8-dev as a dependency
which was removed in llvm-9-tools, so we need to explicitly pull
llvm-9-dev to make a build

src/ci/docker/host-x86_64/x86_64-gnu-llvm-9/Dockerfile

@@ -14,6 +14,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   sudo \
   gdb \
   llvm-9-tools \
+  llvm-9-dev \
   libedit-dev \
   libssl-dev \
   pkg-config \