Fixed crash

Without that line, the sample crashes on my system with the following error:

    OpenCV Error: OpenGL API call (Can't load OpenGL extension [glGenerateMipmap]) in unknown function, file ..\..\..\opencv-2.4.5\modules\core\src\gl_core_3_1.cpp, line 141
    Traceback (most recent call last):
      File "C:\facedetect.py", line 60, in <module>
        cv2.imshow('facedetect', vis)
    cv2.error: ..\..\..\opencv-2.4.5\modules\core\src\gl_core_3_1.cpp:141: error: (-219) Can't load OpenGL extension [glGenerateMipmap]