Fix build after addition of 'empty-cluster.data'

Of course I forgot to add this to TEST_FILES… and only distcheck
catches it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>

Makefile.am

@@ -868,6 +868,7 @@ maninput = \
 
 TEST_FILES = \
 	htest/data/common-suffix.data \
+	htest/data/empty-cluster.data \
 	htest/data/hail-alloc-drbd.json \
 	htest/data/hail-change-group.json \
 	htest/data/hail-invalid-reloc.json \