x86_64-linux-deb9: Install libdw

To be used in DWARF builds.

See ghc!419 (merged).

x86_64-linux-deb9/Dockerfile

@@ -18,6 +18,8 @@ RUN apt-get update -qq && apt-get install --no-install-recommends -qy \
     ca-certificates g++ git make automake autoconf gcc \
     perl python3 texinfo xz-utils lbzip2 patch openssh-client sudo \
     jq wget curl \
+    # DWARF libraries
+    libdw1 libdw-dev \
     # Documentation tools
     python3-sphinx texlive-xetex texlive-latex-extra \
     cabal-install-2.4 ghc-8.4.4 \