Make QNetworkRequest::KnownHeaders a Q_ENUM

... so QDebug e.g. stringyifies it automatically.

Requires making QNetworkRequest a Q_GADGET.

Task-number: QTBUG-114647
Change-Id: Idb4f3ecd98f79c1e28596c7be763898aba4b6562
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>

src/network/access/qnetworkrequest.cpp

@@ -1540,3 +1540,5 @@ QByteArray QNetworkHeadersPrivate::toHttpDate(const QDateTime &dt)
 }
 
 QT_END_NAMESPACE
+
+#include "moc_qnetworkrequest.cpp"

src/network/access/qnetworkrequest.h

@@ -19,6 +19,7 @@ class QHttp1Configuration;
 class QNetworkRequestPrivate;
 class Q_NETWORK_EXPORT QNetworkRequest
 {
+    Q_GADGET
 public:
     enum KnownHeaders {
         ContentTypeHeader,
@@ -35,6 +36,8 @@ public:
         IfMatchHeader,
         IfNoneMatchHeader
     };
+    Q_ENUM(KnownHeaders)
+
     enum Attribute {
         HttpStatusCodeAttribute,
         HttpReasonPhraseAttribute,