Fix namespace build when networkproxy is disabled

The QT_BEGIN_NAMESPACE macro was inside the "#if networkproxy" code
block, whereas the QT_END_NAMESPACE was outside of it.

When networkproxy feature is then disabled, the file will only
effectively have the QT_END_NAMESPACE macro. This fails to compile
when the build is an actual namespace build.

Pick-to: 6.8
Change-Id: Ic0b0fd2d086811664a4f66e2c46424d370641355
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 8d659669b2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/kernel/qnetworkproxy_darwin.cpp

@@ -371,6 +371,6 @@ QList<QNetworkProxy> QNetworkProxyFactory::systemProxyForQuery(const QNetworkPro
     return result;
 }
 
-#endif
-
 QT_END_NAMESPACE
+
+#endif // QT_NO_NETWORKPROXY