modules: don't expose internal variables of FindSDL2.cmake in cache.

modules/FindSDL2.cmake

@@ -80,6 +80,7 @@ if(CORRADE_TARGET_IOS)
     set(_SDL2_FRAMEWORK_LIBRARIES )
     foreach(framework ${_SDL2_FRAMEWORKS})
         find_library(_SDL2_${framework}_LIBRARY ${framework})
+        mark_as_advanced(_SDL2_${framework}_LIBRARY)
         list(APPEND _SDL2_FRAMEWORK_LIBRARIES ${_SDL2_${framework}_LIBRARY})
         list(APPEND _SDL2_FRAMEWORK_LIBRARY_NAMES _SDL2_${framework}_LIBRARY)
     endforeach()