fix panic in LogstreamerInput on Windows

With this configuration on Windows

    [ZST Request Log]
    type = "LogstreamerInput"
    log_directory = "D:/NSX/"
    file_match = "zst_requests.log"

I get the following panic:

    panic: regexp: Compile(`^D:\NSX\zst_requests.log$`): error parsing regexp: invalid escape sequence: `\N`

This pull request escapes the Windows backslashes (and additionally: any other regexp meta characters) in the `log_directory` to create a valid regexp for the `LogstreamerInput`