After commit c7bbd1ae, 'include_directories()' is needed in the CMakeLists.txt…

After commit c7bbd1ae, 'include_directories()' is needed in the CMakeLists.txt (user side). This commit updates the documentation.

doc/tutorials/introduction/linux_gcc_cmake/linux_gcc_cmake.rst

@@ -58,6 +58,7 @@ Now you have to create your CMakeLists.txt file. It should look like this:
    cmake_minimum_required(VERSION 2.8)
    project( DisplayImage )
    find_package( OpenCV REQUIRED )
+   include_directories( ${OpenCV_INCLUDE_DIRS} )
    add_executable( DisplayImage DisplayImage.cpp )
    target_link_libraries( DisplayImage ${OpenCV_LIBS} )