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>
1 job for master in 2 minutes and 6 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #180
build

00:02:06