Fix backslashes in source-map paths on Windows

The path in the source-map always contains backslashes when `--prefix relative` is used. Even if the input files are specified with a forward slash.

Without `--prefix relative`:
```
>uglifyjs --output out.js path/to/a.js --source-map out.js.map
>cat out.js.map
{"version":3,"file":"out.js","sources":["path/to/a.js"],"names":["a"],"mappings":"AAAA,QAASA,KACR,MAAO"}
```

With `--prefix relative`:
```
>uglifyjs --output out.js path/to/a.js --source-map out.js.map --prefix relative
>cat out.js.map
{"version":3,"file":"out.js","sources":["path\\to\\a.js"],"names":["a"],"mappings":"AAAA,QAASA,KACR,MAAO"}
```

This causes an error when debugging in Firefox.