testlib: fix compile error with macosx10.8

[NSDate date] returns an id, so one needs to send a message instead of
accessing a property.

Backport commit 92b3397a from the 5.5 branch.

Change-Id: Id70915e1ac23994a081765e0a527802fef61b573
Reviewed-by: Tim Blechmann <tim@klingt.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

src/testlib/qxctestlogger.mm

@@ -126,7 +126,7 @@ private:
 
     if (!([NSDate timeIntervalSinceReferenceDate] > 0))
         qFatal("error: Device date '%s' is bad, likely set to update automatically. Please correct.",
-            [NSDate date].description.UTF8String);
+            [[NSDate date] description].UTF8String);
 
     XCTestDriver *testDriver = nil;
     if ([QtTestLibWrapper usingTestManager])