Fix link to Tier 2 mingw-builds version

We switched to rev3 in the binary packages to get in a gdb fix.

This change does not apply to Qt 5.

Change-Id: I0328c35671e84e3b592c99cf5ebe8d1cce1ea80f
Reviewed-by: Akseli Salovaara <akseli.salovaara@digia.com>