Ignore dirty working directory for qbs submodule

The submodule is mostly used for building against, not
for editing.

On most cases, its internal status is not interesting,
and git scans it for no reason.

Change-Id: Ic290bd78fdb6704a15264a022125c99cf006a0db
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>