Fix Windows build build with namespaced Qt

This amends 08e1fbbbfa.

Fixes: QTCREATORBUG-22305
Change-Id: Ie929a61e1eefbaafd7e5f9d719dc22426073e2fa
Reviewed-by: hjk <hjk@qt.io>

src/shared/proparser/registry_p.h

@@ -35,6 +35,8 @@
 
 #include <QtCore/qstring.h>
 
+QT_BEGIN_NAMESPACE
+
 namespace QMakeInternal {
 
 /**
@@ -53,3 +55,5 @@ QString qt_readRegistryKey(HKEY parentHandle, const QString &rSubkey,
                            unsigned long options = 0);
 
 }  // namespace QMakeInternal
+
+QT_END_NAMESPACE