Fix: let automake handle CLEAN and DIST

Fix: let automake handle CLEAN and DIST

Files test_seek_empty_packet and test_seek_big_trace are generated by
configuring, not by making.  Therefore, we do not want a make clean to
remove them.  Otherwise, doing...

  * make
  * make clean
  * make
  * make check

... fails.

Automake infers EXTRA_DIST and DISTCLEANFILES from AC_CONFIG_FILES.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>