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]

Please add a test to text_xvlog.vader asserting the required flag is added when the filetype is 'systemverilog'. For example on how to set the filetype in a test check test_cspell.vader 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))