fix Makefile of sdk/node

make clean' will output error due to trying to rm directory lib etc.

Change-Id: If9b712c039409e14e4481b3d2832f658a5d63258
Signed-off-by: xiejunan <xiejunan@huawei.com>

sdk/node/Makefile

@@ -79,4 +79,4 @@ clean:
 	@echo "[CLEAN]"
 	-rm -rf node_modules
 	-rm -rf doc
-	-find lib | grep -v "protos/google" | grep -v "hash.js" | xargs rm
+	-find lib -type f | grep -v "protos/google" | grep -v "hash.js" | xargs rm