Guard xmlstream header in a source-compatible way

Using QT_REQUIRE_CONFIG results in a static_assert if the xmlstream
feature is not available. This is a SiC change, as the user has
no reasonable ways to guard against it.
Fix it by using

 if QT_CONFIG(xmlstream)

instead.

This commit amends 7337474d04

Pick-to: 6.6
Change-Id: I0c55e4cff06157743c05a543a092f9be1eb67c2d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

src/corelib/serialization/qxmlstream.h

@@ -6,7 +6,7 @@
 
 #include <QtCore/qiodevice.h>
 
-QT_REQUIRE_CONFIG(xmlstream);
+#if QT_CONFIG(xmlstream)
 
 #include <QtCore/qlist.h>
 #include <QtCore/qscopedpointer.h>
@@ -418,4 +418,6 @@ private:
 
 QT_END_NAMESPACE
 
+#endif // feature xmlstream
+
 #endif // QXMLSTREAM_H