sdbus++-gendir: avoid duplicate filenames with --list-all
sdbus++-gendir: avoid duplicate filenames with --list-all

With the `--list-all` option, the program can end up emitting the
filename for output files multiple times.  This can cause issues with
typical use cases of utilizing sdbus++-gendir as part of meson, because
a plain usage of this tool may end up causing duplicated build actions.
In the conversion to meson in phosphor-dbus-interfaces it was noted that
the same markdown files could be installed twice.

Create a hash table to keep track of which filenames have been emitted
and prevent duplicate emission.

Signed-off-by: Patrick Williams <cbb7353e6d953ef360baf960c122346276c6e320@stwcx.xyz>
Change-Id: I1d8d69c6df3ec9b13acfe955f8cc3e81d8748033

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus