Compile with -no-opengl

QCocoaBackingstore::toImage() can only be Q_DECL_OVERRIDE
if QPlatformBackingStore::toImage() is present, which
it isn’t for NO_OPENGL builds.

Change-Id: Ib116f40fd26defb29a8d520d3e3fb104d8da8d57
Task-number: QTBUG-51694
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>