CMakeLists.txt: add src/joystick/iphoneos/*.m to Darwin joystick sources

.. so that there won't be missing symbols.
TODO: add checks for SDL_JOYSTICK_MFI ???

CMakeLists.txt

@@ -1652,6 +1652,8 @@ elseif(APPLE)
       set(HAVE_SDL_SENSORS 1)
     else()
       file(GLOB JOYSTICK_SOURCES ${SDL2_SOURCE_DIR}/src/joystick/darwin/*.c)
+      file(GLOB JOYSTICK_SOURCES ${SDL2_SOURCE_DIR}/src/joystick/iphoneos/*.m)
+      # FIXME: add checks for SDL_JOYSTICK_MFI???
       set(SDL_JOYSTICK_IOKIT 1)
       set(SDL_FRAMEWORK_IOKIT 1)
       set(SDL_FRAMEWORK_FF 1)