add the time as a string

AircraftReport.proto

@@ -10,5 +10,5 @@ message AircraftReport {
     Coordinate position           = 3; // current WGS84 position
     Dynamics   dynamics           = 4; // current aircraft dynamics
     string     destination        = 5; // the destination airport
-    Time       timeAtIAF          = 6; // UTC time when aircraft is at IAF
+    string     timeAtIAF          = 6; // UTC time when aircraft is at IAF
 }