Add missing include in tst_QHttpHeaders

A QMultiMap is used, while only qset.h is included.
FTBFS w/o precompiled headers.

Include qmap.h as well.

Pick-to: 6.6 6.5
Change-Id: I5f6e1d204bfa94cc6a81f6883c7a7b0ee1e9963c
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 84b792fa98b3a21119f16b0cd9443f90b1ad504d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/network/access/qhttpheaders/tst_qhttpheaders.cpp

@@ -5,6 +5,7 @@
 
 #include <QtTest/qtest.h>
 
+#include <QtCore/qmap.h>
 #include <QtCore/qset.h>
 
 using namespace Qt::StringLiterals;