Fix encoding of non-opened include files.

The old implementation readed the file and converted the QString toUtf8,
which seems wrong. Now use Creators default encoding.

This fixes at least wrong macro offsets that leaded to highlighting
errors in Find Usages, if there were non-ASCII characters before the
macro definition.

This should also partially solve QTCREATORBUG-7122.

Change-Id: Ic4a5add5f4769bd3d5b62fc2d67598e7abf352d9
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>