pmdawindows: mark sqlserver.user_settable.query as optional
pmdawindows: mark sqlserver.user_settable.query as optional

Resolves a warning in pmcd.log about this metric noticed by
lootic on IRC.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp