QOperatingSystemVersion: fix documentation for the Unexported entries

They were not showing up because the qdoc branch sees them as private.

Pick-to: 6.8
Change-Id: I8be18c389288ff9b589267dd17fbc16dcd7998c7
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/global/qoperatingsystemversion.h

@@ -138,6 +138,7 @@ public:
 class QOperatingSystemVersion : public QOperatingSystemVersionBase
 {
     using QOperatingSystemVersionUnexported = QOperatingSystemVersionBase;
+public:
 #endif
 
     // ### Qt7: Regroup with the rest below