/usr/lib/x86_64-linux-gnu/libsl.a is in heimdal-dev 1.6~git20120311.dfsg.1-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/heimdal/libsl.a
/usr/lib/x86_64-linux-gnu/heimdal/libsl.a