Update xvlog.vim

[zhangt123456]

when using xvlog in Vivado 2019.1 for SystemVerilog, you should add the option '-sv'.

Where are the tests? Have you added tests? Have you updated the tests? Read the comment above and the documentation referenced in it first. Write tests!

Seriously, read :help ale-dev and write tests.

[hsanson]

Also there is a linter error:

ale_linters/verilog/xvlog.vim:6:14: Use robust operators ==# or ==? instead of == (see Google VimScript Style Guide (Matching))

[stale[bot]]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests. If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.