Add: [NewGRF] Inspection window for airports.

As as the station window combines all station types, accessing the
debug view is via the parent of the airport tile only.

(cherry picked from commit a6f2f3c042)

src/newgrf_debug_gui.cpp

@@ -32,6 +32,7 @@
 #include "train.h"
 #include "roadveh.h"
 
+#include "newgrf_airport.h"
 #include "newgrf_airporttiles.h"
 #include "newgrf_debug.h"
 #include "newgrf_object.h"