Fix constant false comparison of out-of-range enums

QGraphicsItem::GraphicsItemFlag is unsigned, so a comparison to -1
is always false.

qgraphicsitem.cpp:847:39: error: comparison of constant -1 with expression of type 'QGraphicsItem::GraphicsItemFlag' is always false [-Werror,-Wtautological-constant-out-of-range-compare]

Change-Id: I3fc59b777d09060dd34e81f51ed8bdf41354a0f1
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>