QTzTimeZoneCache: make findEntry() static

It doesn't need access to the members, so making it static both saves
the implicit this pointer, and communicates to the reader that this is
function doesn't access other QTzTimeZoneCache members.

In particular, it proves that it doesn't really need to be called with
m_mutex locked, cf. QTBUG-122138.

Task-number: QTBUG-122138
Pick-to: 6.6 6.5
Change-Id: I28c99be8d066c1647aaf7aa256b6400a6dd92955
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 7862453ba9cbcb601b9aad59dd821ae49661de4a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/time/qtimezoneprivate_tz.cpp

@@ -783,7 +783,7 @@ public:
     QTzTimeZoneCacheEntry fetchEntry(const QByteArray &ianaId);
 
 private:
-    QTzTimeZoneCacheEntry findEntry(const QByteArray &ianaId);
+    static QTzTimeZoneCacheEntry findEntry(const QByteArray &ianaId);
     QCache<QByteArray, QTzTimeZoneCacheEntry> m_cache;
     QMutex m_mutex;
 };