Fix bug in distclean which removes needed files

Running distclean removes the following files because of an error
in Makefile.am

deleted:    tests/zfs-tests/include/commands.cfg
deleted:    tests/zfs-tests/include/libtest.shlib
deleted:    tests/zfs-tests/include/math.shlib
deleted:    tests/zfs-tests/include/properties.shlib
deleted:    tests/zfs-tests/include/zpool_script.shlib

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: David Quigley <david.quigley@intel.com>
Closes openzfs#6636

tests/zfs-tests/include/Makefile.am

@@ -10,4 +10,4 @@ dist_pkgdata_SCRIPTS = \
 EXTRA_DIST=default.cfg.in
 
 distclean-local::
-	-$(RM) $(dist_pkgdata_SCRIPTS)
+	-$(RM) default.cfg