GPS_LEAPSECONDS_MILLIS gives us wrong system time on RPi Navio2 NMEA GPS

#### Issue details
_Please describe the problem, or desired feature_
// the number of GPS leap seconds
#define GPS_LEAPSECONDS_MILLIS 18000ULL
We are getting wrong system time set on navio2 on RPi with an NMEA based GPS.

If we change the above to
#define GPS_LEAPSECONDS_MILLIS 15000ULL
Then the system time will be correct

#### Version
_What version was the issue encountered with_
Latest version

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[X ] Rover
[ ] Submarine

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_
Boat
#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_
Navio2 on RPi with AIRMAR NMEA2K GPS

#### Logs
_Please provide a link to any relevant logs that show the issue_