Recognize the -C option as an alias of --css

[CodeLongAndProsper90]

Currently, wlogout --help specifies -C as an alias of --css but then exits with

$ nix-shell -p wlogout --run "wlogout -C ~/.dots/home/config/display/wlogout/style.css"
wlogout: invalid option -- 'C'

This changes the option string passed to getopt_long to recognize -C as an alias of --css.