QHttpHeaders: add a missing detach()

Before calling a mutable member function (in this case,
QList::reserve()) on one of the Private members, we need to ensure
exclusive ownership of our external state, otherwise we have a Data
Race, because QList::reserve() is not thread-safe (only re-entrant).

Change-Id: I1249de9449865cbf6ab519b0d38b3bf4e743f16d
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
(cherry picked from commit 119a6b51869b7ffba4fa49bc07e78a243fe4a9ba)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/access/qhttpheaders.cpp

@@ -1040,6 +1040,7 @@ qsizetype QHttpHeaders::size() const noexcept
 */
 void QHttpHeaders::reserve(qsizetype size)
 {
+    d.detach();
     d->headers.reserve(size);
 }