QHttpMultiPartPrivate: de-inline dtors of polymorphic classes

Prevents weak (= duplicated) vtables and the associated problems.

Amends the start of the public history.

Pick-to: 6.7 6.5
Task-number: QTBUG-45582
Change-Id: I26618483900437aca05c753a5d53a359e43bd702
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 645509935ad5a2e8a3fcc858c41d08919a2596bd)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/access/qhttpmultipart.cpp

@@ -409,6 +409,14 @@ QHttpMultiPartPrivate::QHttpMultiPartPrivate() : contentType(QHttpMultiPart::Mix
     Q_ASSERT(boundary.size() <= 70);
 }
 
+QHttpMultiPartPrivate::~QHttpMultiPartPrivate()
+{
+    delete device;
+}
+
+QHttpMultiPartIODevice::~QHttpMultiPartIODevice()
+    = default;
+
 qint64 QHttpMultiPartIODevice::size() const
 {
     // if not done yet, we calculate the size and the offsets of each part,

src/network/access/qhttpmultipart_p.h

@@ -94,8 +94,7 @@ public:
             QIODevice(), multiPart(parentMultiPart), readPointer(0), deviceSize(-1) {
     }
 
-    ~QHttpMultiPartIODevice() {
-    }
+    ~QHttpMultiPartIODevice() override;
 
     virtual bool atEnd() const override {
         return readPointer == size();
@@ -135,11 +134,7 @@ class QHttpMultiPartPrivate: public QObjectPrivate
 public:
 
     QHttpMultiPartPrivate();
-
-    ~QHttpMultiPartPrivate()
-    {
-        delete device;
-    }
+    ~QHttpMultiPartPrivate() override;
 
     QList<QHttpPart> parts;
     QByteArray boundary;