Fixed 'ImportError: failed to find libmagic.  Check your installation' when using cygwin's python

When using cygwin, and python distribution, that comes with it, `sys.platform` is equal to "cygwin", rather than "win32".
So I made this value be treated the same in magic.py

Test Plan:
    $ /bin/python
    >>> import magic
    >>> # Ensure no error message displayed
    >>> exit()