GCS_MAVLink: make send_text and send_textv const

libraries/GCS_MAVLink/GCS.h

@@ -121,8 +121,8 @@ public:
     void        init(AP_HAL::UARTDriver *port, mavlink_channel_t mav_chan);
     void        setup_uart(const AP_SerialManager& serial_manager, AP_SerialManager::SerialProtocol protocol, uint8_t instance);
     void        send_message(enum ap_message id);
-    void        send_text(MAV_SEVERITY severity, const char *fmt, ...);
-    void        send_textv(MAV_SEVERITY severity, const char *fmt, va_list arg_list);
+    void        send_text(MAV_SEVERITY severity, const char *fmt, ...) const;
+    void        send_textv(MAV_SEVERITY severity, const char *fmt, va_list arg_list) const;
     void        queued_param_send();
     void        queued_mission_request_send();
     // packetReceived is called on any successful decode of a mavlink message

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -686,13 +686,13 @@ void GCS_MAVLINK::handle_param_value(mavlink_message_t *msg)
     mount->handle_param_value(msg);
 }
 
-void GCS_MAVLINK::send_textv(MAV_SEVERITY severity, const char *fmt, va_list arg_list)
+void GCS_MAVLINK::send_textv(MAV_SEVERITY severity, const char *fmt, va_list arg_list) const
 {
     char text[MAVLINK_MSG_STATUSTEXT_FIELD_TEXT_LEN+1];
     hal.util->vsnprintf(text, sizeof(text), fmt, arg_list);
     gcs().send_statustext(severity, (1<<chan), text);
 }
-void GCS_MAVLINK::send_text(MAV_SEVERITY severity, const char *fmt, ...)
+void GCS_MAVLINK::send_text(MAV_SEVERITY severity, const char *fmt, ...) const
 {
     va_list arg_list;
     va_start(arg_list, fmt);