Ignore artefacts from sampledata

9b5ae5f53e introduced the creation of some symlinks to avoid issues on
Cygwin. "make" now leaves the repository in a dirty state since we
forgot to exclude those symlinks.

The symlinks in question are products of the build/test process so we
need to avoid commiting them.

Signed-off-by: Gabriel Filion <gabster@lelutin.ca>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>