AP_LeakDetector: add missing override keywords

libraries/AP_LeakDetector/AP_LeakDetector_Analog.h

@@ -6,7 +6,7 @@
 class AP_LeakDetector_Analog : public AP_LeakDetector_Backend {
 public:
     AP_LeakDetector_Analog(AP_LeakDetector &_leak_detector, AP_LeakDetector::LeakDetector_State &_state);
-    void read(void);
+    void read(void) override;
 
 private:
     AP_HAL::AnalogSource *source;

libraries/AP_LeakDetector/AP_LeakDetector_Digital.h

@@ -5,5 +5,5 @@
 class AP_LeakDetector_Digital : public AP_LeakDetector_Backend {
 public:
     AP_LeakDetector_Digital(AP_LeakDetector &_leak_detector, AP_LeakDetector::LeakDetector_State &_state);
-    void read(void);
+    void read(void) override;
 };