add requirements for python ctypes

[LIU Yulong]

Swift source code install

env:
python2.6

ctypes: 1.1.0

object-server error:

Traceback (most recent call last):
  File "/usr/bin/swift-object-server", line 20, in <module>
    from swift.obj import server
  File "/home/yulong/swift/swift/obj/server.py", line 51, in <module>
    from swift.obj.diskfile import DATAFILE_SYSTEM_META, DiskFileManager
  File "/home/yulong/swift/swift/obj/diskfile.py", line 60, in <module>
    from swift.common.splice import splice, tee
  File "/home/yulong/swift/swift/common/splice.py", line 101, in <module>
    tee = Tee()
  File "/home/yulong/swift/swift/common/splice.py", line 53, in __init__
    c_tee.restype = ctypes.c_ssize_t
AttributeError: 'module' object has no attribute 'c_ssize_t'

[LIU Yulong (dragon889)]

Related commit:
https://review.openstack.org/#/c/135319/

[LIU Yulong (dragon889)]

Do this:
git reset --hard f48350865ec1666fd51bf161c339a90aee9ff7cd
(https://github.com/openstack/swift/commit/8bbb50db058b1aa7278f00f36fd9e468a0cd9f6f)

finally everything works fine.

[LIU Yulong (dragon889)]

It seems that swift python2.6 support will be deprecated.