Fix submodule branch

Signed-off-by: Li, Xun <xun.li@intel.com>

Author: llly

.gitmodules

@@ -16,7 +16,7 @@
 [submodule "external/protobuf/protobuf_code"]
 	path = external/protobuf/protobuf_code
 	url = https://github.com/protocolbuffers/protobuf.git
-	branch = v3.20.1
+	branch = 3.20.x
 [submodule "external/sgx-emm/emm_src"]
 	path = external/sgx-emm/emm_src
 	url = https://github.com/intel/sgx-emm.git