Cocoa: initialize QCocoaEventDispacherPrivate::lastSerial

This silences warnings from valgrind about a conditional depending on an
uninitialized value.

Change-Id: I819a44ed5dc02e163c00849811870c94f66b6651
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>

src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm

@@ -895,6 +895,7 @@ QCocoaEventDispatcherPrivate::QCocoaEventDispatcherPrivate()
       nsAppRunCalledByQt(false),
       cleanupModalSessionsNeeded(false),
       currentModalSessionCached(0),
+      lastSerial(-1),
       interrupt(false)
 {
 }