MAVLinkProtocol: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

src/comm/MAVLinkProtocol.cc

@@ -383,7 +383,7 @@ QString MAVLinkProtocol::getName()
 }
 
 /** @return System id of this application */
-int MAVLinkProtocol::getSystemId()
+int MAVLinkProtocol::getSystemId() const
 {
     return systemId;
 }

src/comm/MAVLinkProtocol.h

@@ -47,7 +47,7 @@ public:
     /** @brief Get the human-friendly name of this protocol */
     QString getName();
     /** @brief Get the system id of this application */
-    int getSystemId();
+    int getSystemId() const;
     /** @brief Get the component id of this application */
     int getComponentId();
 
@@ -60,7 +60,7 @@ public:
         return MAVLINK_VERSION;
     }
     /** @brief Get the currently configured protocol version */
-    unsigned getCurrentVersion() {
+    unsigned getCurrentVersion() const{
         return _current_version;
     }
     /**