[docbook-css] Support for sgmltag

[martin.gautier@myrnham.co.uk]

To support the <sgmltag> element, I have added "sgmltag" to the following 
line entry in styles.css

cmdsynopsis, code, command, computeroutput, envar, filename, keycode, 
keysym,
literal, option, parameter, systemitem, sgmltag {
        font-family: monospace;
}