AP_Arming: make gps-checks virtual

Copter's GPS checks will override this

libraries/AP_Arming/AP_Arming.h

@@ -92,7 +92,7 @@ protected:
 
     virtual bool compass_checks(bool report);
 
-    bool gps_checks(bool report);
+    virtual bool gps_checks(bool report);
 
     bool battery_checks(bool report);