install libelf-dev

To try to avoid this message during build:
warning: Cannot use CONFIG_STACK_VALIDATION=y, please install libelf-dev, libelf-devel or elfutils-libelf-devel
<https://gitlab.com/freifunk-region/stuttgart/site/-/jobs/200400306>

Dockerfile

 FROM debian:9
 
 RUN apt-get update \
-  && apt-get install -y git subversion python build-essential gawk unzip libz-dev libncurses-dev libssl-dev time curl wget ecdsautils \
+  && apt-get install -y git subversion python build-essential gawk unzip libelf-dev libz-dev libncurses-dev libssl-dev time curl wget ecdsautils \
   && apt-get clean
 
 # gluon build fails when running as root