Codechange: fix Intel C++ Compiler linking issues.

GetAircraftFlightLevel<Aircraft> is only used in static functions
inside aircraft_cmd.cpp. With GCC, Clang and MSVC this is not an
issue, but on ICC fails linking, because it doesn't find this
version of this template. Possibly these two pieces of information
are linked.
Explicit defining the function fixes the issue.

src/aircraft_cmd.cpp

@@ -783,6 +783,7 @@ int GetAircraftFlightLevel(T *v, bool takeoff)
 }
 
 template int GetAircraftFlightLevel(DisasterVehicle *v, bool takeoff);
+template int GetAircraftFlightLevel(Aircraft *v, bool takeoff);
 
 /**
  * Find the entry point to an airport depending on direction which