fix export -s

Resolves:
stg export -s `stg top`
Traceback (most recent call last):
  File "/usr/local/lib/python2.5/site-packages/stgit/main.py", line 152, in _main
    ret = command.func(parser, options, args)
  File "/usr/local/lib/python2.5/site-packages/stgit/commands/export.py", line 169, in func
    f = sys.stdout
NameError: global name 'sys' is not defined

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>