matlab: fix bindings generation for xfeatures2d

This PR aims to fix the problem with Matlab bindings generation for xfeatures2d. The latter fails as gen_matlab.py fails to read the module's header due to incorrectly assuming it to be ${CMAKE_SOURCE_DIR}/modules/xfeatures2d/include/opencv2/xfeatures2d.hpp.

Therefore, gen_matlab.py script is extended to accept multiple values for --modules-root argument, and to check for a module's header in all possible paths. This way, both ${CMAKE_SOURCE_DIR}/modules and
${OPENCV_EXTRA_MODULES_PATH} can be passed as modules root paths, and the main header for xfeatures2d module can be correctly located.