command: Fix except declaration for get_comedi_cmd_pointer

command: Fix except declaration for get_comedi_cmd_pointer

Avoid:

  Error compiling Cython file:
  ------------------------------------------------------------
  ...

  cdef class Command (object):
      cdef _comedi_h.comedi_cmd _cmd
      cdef public list _fields

      cdef _comedi_h.comedi_cmd *get_comedi_cmd_pointer(self)
                               ^
  ------------------------------------------------------------

  pycomedi/command.pxd:26:30: C method 'get_comedi_cmd_pointer' is declared but not defined

Cython is stricter about that sort of thing since 0.22 (2015-02-11)
[1]:

  Mismatching 'except' declarations on signatures in .pxd and .pyx
  files failed to produce a compile error.

[1]: https://github.com/cython/cython/blob/master/CHANGES.rst#022-2015-02-11