make-windows-release: allow *.cmd files to operate from paths w/ spaces

If wimlib is unpacked in a path with spaces in it, such as
"C:\Documents and Settings" (default user location on Windows XP), the
shell will prompt back with a message like:

    'C:\Documents' is not recognized as an internal or external command,
    operable program or batch file.

Simply using double quotes as a guard resolves this and allows it to work.