Fix helipad runtime type definition

This broke various places trying to detect helipads vs runways.

src/Airports/runways.cxx

@@ -222,7 +222,7 @@ FGHelipad::FGHelipad(PositionedID aGuid,
                         const double heading, const double length,
                         const double width,
                         const int surface_code) :
-  FGRunwayBase(aGuid, RUNWAY, aIdent, aGeod,
+  FGRunwayBase(aGuid, HELIPAD, aIdent, aGeod,
                heading, length, width, surface_code)
 {
 }