Add missing compile definition for FLUKA usage

WITH_FLUKA has not been propagated to c8_air_shower.cpp. So even if CMake said that it would compile with FLUKA, it actually compiled with UrQMD...

applications/CMakeLists.txt

 SET(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
 
+add_executable (c8_air_shower c8_air_shower.cpp)
+target_link_libraries (c8_air_shower CORSIKA8)
+
 if(WITH_FLUKA)
   message("compiling c8_air_shower.cpp with FLUKA")
+  target_compile_definitions(c8_air_shower PRIVATE WITH_FLUKA)
 else()
   message("compiling c8_air_shower.cpp with UrQMD")
 endif()
-add_executable (c8_air_shower c8_air_shower.cpp)
-target_link_libraries (c8_air_shower CORSIKA8)
 
 install (
   TARGETS c8_air_shower DESTINATION bin