DataFlash: add override keyword

libraries/DataFlash/DFMessageWriter.h

@@ -24,8 +24,8 @@ protected:
 class DFMessageWriter_WriteSysInfo : public DFMessageWriter {
 public:
 
-    void reset();
-    void process();
+    void reset() override;
+    void process() override;
 
 private:
     enum write_sysinfo_blockwriter_stage {
@@ -40,8 +40,8 @@ private:
 class DFMessageWriter_WriteEntireMission : public DFMessageWriter {
 public:
 
-    void reset();
-    void process();
+    void reset() override;
+    void process() override;
 
     void set_mission(const AP_Mission *mission);
 
@@ -66,14 +66,14 @@ public:
         {
         }
 
-    virtual void set_dataflash_backend(class DataFlash_Backend *backend) {
+    virtual void set_dataflash_backend(class DataFlash_Backend *backend) override {
         DFMessageWriter::set_dataflash_backend(backend);
         _writesysinfo.set_dataflash_backend(backend);
         _writeentiremission.set_dataflash_backend(backend);
     }
 
-    void reset();
-    void process();
+    void reset() override;
+    void process() override;
     bool fmt_done() { return _fmt_done; }
 
     void set_mission(const AP_Mission *mission);