debian/triggers: add (to run ldconfig) (#664)

After installing the deb package, st-util fails to start:

> $ st-util
> st-util: error while loading shared libraries: libstlink-shared.so.1:
> cannot open shared object file: No such file or directory

The reason is, no one ran `ldconfig` upon installing the library.

The solution, as per latest Debian recommendations [1], is to add an
ldconfig trigger in debian/triggers, which is what this commit does.

[1] https://www.debian.org/doc/debian-policy/#ldconfig



Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>