AP_Beacon: added const

libraries/AP_Beacon/AP_Beacon.cpp

@@ -109,13 +109,13 @@ void AP_Beacon::init(void)
 }
 
 // return true if beacon feature is enabled
-bool AP_Beacon::enabled(void)
+bool AP_Beacon::enabled(void) const
 {
     return (_type != AP_BeaconType_None);
 }
 
 // return true if sensor is basically healthy (we are receiving data)
-bool AP_Beacon::healthy(void)
+bool AP_Beacon::healthy(void) const
 {
     if (!device_ready()) {
         return false;

libraries/AP_Beacon/AP_Beacon.h

@@ -59,10 +59,10 @@ public:
     void init(void);
 
     // return true if beacon feature is enabled
-    bool enabled(void);
+    bool enabled(void) const;
 
     // return true if sensor is basically healthy (we are receiving data)
-    bool healthy(void);
+    bool healthy(void) const;
 
     // update state of all beacons
     void update(void);