common.xml: add GRIPPER_ACTION_STOP message ([mavlink#2342](mavlink#2342))

Author: neo-0007

message_definitions/v1.0/common.xml

@@ -588,6 +588,9 @@
       <entry value="1" name="GRIPPER_ACTION_GRAB">
         <description>Gripper grab onto cargo.</description>
       </entry>
+      <entry value="2" name="GRIPPER_ACTION_STOP">
+        <description>Gripper stop (maintain current grip position).</description>
+      </entry>
     </enum>
     <!-- winch action enum -->
     <enum name="WINCH_ACTIONS">