drop libdl dependency
Our nightly package build says:
[ 222s] dpkg-shlibdeps: warning: package could avoid a useless dependency if debian/osmo-trx/usr/bin/osmo-trx was not linked against libdl.so.2 (it uses none of the library's symbols)
If that is accurate, maybe we should drop the dependency. I can't see any libdl though, only libdbd-sqlite3 in debian/control, and it appears that that can be dropped.