Docker: T5286: Kernel depends on libelf-dev - add missing dependency

Commit dd194c6e8 ("Docker: T5286: remove XDP support") dropped all relevant
package build dependencies for XDP - which is good as we should not carry
leftovers.

The Linux Kernel dependency list in contrast always lacked this package. Re-add
the package to the appropriate section.

docker/Dockerfile

@@ -183,6 +183,7 @@ RUN wget https://salsa.debian.org/klausenbusk-guest/debootstrap/commit/a9a603b17
 RUN apt-get update && apt-get install -y \
       gnupg2 \
       rsync \
+      libelf-dev \
       libncurses5-dev \
       flex \
       bison \