lang/expect: register LDCONFIG path

Because expect installs its shared libraries as symlinks

libexpect.so.1 -> lib/expect5.45.4/libexpect5.45.4.so

programs and libraries linked with -lexpect will fail to locate the final .so
file.

See
https://lists.freebsd.org/pipermail/freebsd-tcltk/2020-February/000677.html

Reported by: Daniel Morante <daniel@morante.net>