find_package looks for PackageName_ROOT and the package is now
named 'protobuf'.
Fixes: QTBUG-132901
Pick-to: 6.9
Change-Id: Ifb956e4ab52c3df059d2624eb989c97473a662a2
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>