add rule for clean (#4364)

* add rule for clean

* Update clean rule

Seems like lib/ directory is not made by the makefile
So don't delete directory, just the contents of it.

apps/howto_deploy/Makefile

@@ -49,3 +49,5 @@ lib/cpp_deploy_pack: cpp_deploy.cc lib/test_addone_sys.o lib/libtvm_runtime_pack
 lib/cpp_deploy_normal: cpp_deploy.cc lib/test_addone_sys.o
 	@mkdir -p $(@D)
 	$(CXX) $(PKG_CFLAGS) -o $@  $^ -ltvm_runtime $(PKG_LDFLAGS)
+clean:
+	rm lib/libtvm_runtime_pack.o lib/test_addone_sys.o lib/cpp_deploy_pack lib/cpp_deploy_normal