AP_GPS: make time_epoch_usec const

libraries/AP_GPS/AP_GPS.cpp

@@ -346,7 +346,7 @@ uint64_t AP_GPS::time_epoch_convert(uint16_t gps_week, uint32_t gps_ms)
 /**
    calculate current time since the unix epoch in microseconds
  */
-uint64_t AP_GPS::time_epoch_usec(uint8_t instance)
+uint64_t AP_GPS::time_epoch_usec(uint8_t instance) const
 {
     const GPS_State &istate = state[instance];
     if (istate.last_gps_time_ms == 0) {

libraries/AP_GPS/AP_GPS.h

@@ -373,8 +373,8 @@ public:
     void send_blob_update(uint8_t instance);
 
     // return last fix time since the 1/1/1970 in microseconds
-    uint64_t time_epoch_usec(uint8_t instance);
-    uint64_t time_epoch_usec(void) {
+    uint64_t time_epoch_usec(uint8_t instance) const;
+    uint64_t time_epoch_usec(void) const {
         return time_epoch_usec(primary_instance);
     }