v4l2_ext_control needs _pack_ attribute

[Pete Eberlein]

In the videodev2.h header, the struct v4l2_ext_control has __attribute__ ((packed));
and this needs to be reflected in the class v4l2_ext_control ctypes.Structure by adding an attribute _pack_ = True
like this:

class v4l2_ext_control(ctypes.Structure):
    class _u(ctypes.Union):
        _fields_ = [
            ('value', ctypes.c_int32),
            ('value64', ctypes.c_int64),
            ('reserved', ctypes.c_void_p),
        ]

    _fields_ = [
        ('id', ctypes.c_uint32),
        ('reserved2', ctypes.c_uint32 * 2),
        ('_u', _u)
    ]

    _anonymous_ = ('_u',)

    _pack_ = True

[Pete Eberlein (pete-eberlein)]

Also, _pack_ should probably also be added to the classes v4l2_register and v4l2_chip_ident as well.