docker joystream-node: add llvm dep

joystream-node.Dockerfile

@@ -3,7 +3,7 @@ RUN rustup component add rustfmt clippy
 RUN rustup install nightly-2021-02-20 --force
 RUN rustup target add wasm32-unknown-unknown --toolchain nightly-2021-02-20
 RUN apt-get update && \
-  apt-get install -y curl git gcc xz-utils sudo pkg-config unzip clang libc6-dev
+  apt-get install -y curl git gcc xz-utils sudo pkg-config unzip clang llvm libc6-dev
 
 FROM rustup AS builder
 LABEL description="Compiles all workspace artifacts"