lttng-modules: add 'libelf' to fix build against linux 4.14

Fixes this:

$ nix-build -A linuxPackages.lttng-modules
[...]
/nix/store/...-linux-4.14.48-dev/lib/modules/4.14.48/source/Makefile:948: \
  *** "Cannot generate ORC metadata for CONFIG_UNWINDER_ORC=y, please install libelf-dev, libelf-devel or elfu

(Linux 4.16+ has other issues, so mark as broken.)