QGraphicsFrameCaptureMetal: silence deprecation warnings

StringBuilder has deprecated support for char[], using wide string will
also avoid run-time unicode conversions.

Change-Id: Iff865463fb255acfa55224393807ece9ecb818b4
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
Reviewed-by: Hatem ElKharashy <hatem.elkharashy@qt.io>
(cherry picked from commit d862242)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/util/qgraphicsframecapturemetal.mm

@@ -157,7 +157,9 @@
 
 void QGraphicsFrameCaptureMetal::updateCaptureFileName()
 {
-    m_traceURL = QUrl::fromLocalFile(m_capturePath + "/" + m_capturePrefix + "_" + QString::number(frameNumber) + ".gputrace").toNSURL();
+    m_traceURL = QUrl::fromLocalFile(m_capturePath + u"/" + m_capturePrefix + u"_"
+                                     + QString::number(frameNumber) + u".gputrace")
+                         .toNSURL();
     // We need to remove the trace file if it already existed else MTLCaptureManager
     // will fail to.
     if ([NSFileManager.defaultManager fileExistsAtPath:m_traceURL.path])