iop: add "libelf-dev" build dependency
Resolves an issue where some packages are unable to build with an error message similar to the one below:
make[9]: *** No rule to make target '/home/dev/iopsyswrt/build_dir/target-x86_64_musl/linux-x86_64/xtables-addons-3.21/extensions/ACCOUNT/xt_ACCOUNT.o', needed by '/home/dev/iopsyswrt/build_dir/target-x86_64_musl/linux-x86_64/xtables-addons-3.21/extensions/ACCOUNT/xt_ACCOUNT.mod'. Stop.
In particular, this should fix the x86 Docker image build.
src: https://forum.openwrt.org/t/solved-problem-compiling-22-03-4/157221