added syscall message definition to protobuffer

Change-Id: Ieec0e66c836799364c0171f06ac78284c0389da2
diff --git a/proto/AndroidSystemControlMessage.proto b/proto/AndroidSystemControlMessage.proto
index 63bf798..d61e3d3 100644
--- a/proto/AndroidSystemControlMessage.proto
+++ b/proto/AndroidSystemControlMessage.proto
@@ -105,7 +105,7 @@
   // for LIST_APIS
   // none
 
-  // for CALL_API
+  // for CALL_API and VTS_AGENT_COMMAND_INVOKE_SYSCALL
   optional bytes arg = 4001;
 
   // for VTS_AGENT_COMMAND_EXECUTE_SHELL_COMMAND