This is mandatory in public headers (qiodevice.h, qopengl*, etc.), but
it's a good idea even in private headers, in case someone includes
that header first somewhere. In particular, all platformsupport API is
private.

Change-Id: If287baa5d9ed14e93c1666efa0e6332c4c1cd9a4
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>