Fix build error on gcc 7+ for implicit switch statement fallthroughs
Fix build error on gcc 7+ for implicit switch statement fallthroughs

Change-Id: I48632a42892d8e9a616255fe106e40161b0ee55f
Signed-off-by: Robert Hughes <3be7ff6f70e31aa390087d7c66897beeb25aa6c5@arm.com>

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn