Updated xobjdetect module CMakeLists.txt

Currently if you are building OpenCV with contrib under ios, not as framework, this can leads to errors in config.
I suppose it would be better if xobjdetect module app will be optional with the BUILD_opencv_apps just like others tools in OpenCV

modules/xobjdetect/CMakeLists.txt

 set(the_description "Object detection algorithms")
 ocv_define_module(xobjdetect opencv_core opencv_imgproc opencv_highgui opencv_objdetect WRAP python)
-if (NOT APPLE_FRAMEWORK)
+if (BUILD_opencv_apps AND NOT APPLE_FRAMEWORK)
     add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/tools ${CMAKE_CURRENT_BINARY_DIR}/tools)
 endif()