[Mesa-stable] Fix windows builds

[Roland Scheidegger]

Unfortunately 7e61b44dcd6175579f60d8ff2f703a6c83e33d27 which fixed a
rather insignificant bug caused all windows based builds to fail.
The next commit bba8f10598866776ae198b363b3752c2e3bbb126 fixed this but
it was not sent to mesa-stable, therefore I'm nominating this for the
stable branches (same as the breaking commit, so 9.0, 9.1, 9.2, 10.0).

Roland