Fix missing ending semicolon in UglifyJS output

The fix is directly taken from the UglifyJS [source code](https://github.com/mishoo/UglifyJS/blob/master/bin/uglifyjs#L257). It has to be taken care of in smoosh's code too because smoosh directly uses the UglifyJS v1 API.

In UglifyJS, the semicolon is also added after the [gen_code(ast)](https://github.com/mishoo/UglifyJS/blob/master/bin/uglifyjs#L315) call.