Minor fix to the "clean" Makefile target.

Review URL: http://codereview.chromium.org/549223
diff --git a/tests/Makefile b/tests/Makefile
index 8fcd21f..7e2601d 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -20,4 +20,4 @@
 digest_utility.o:	digest_utility.c
 	$(CC) $(CFLAGS) $(INCLUDES) -c $< -o $@ 
 clean:
-	rm -f $(OBJS) sha_tests verify_data
+	rm -f $(OBJS) sha_tests verify_data signature_digest