testlib: fix compile error with macosx10.8

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

Change-Id: I21ce9b64310315accb7a89278b292dd5c73adc4d
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])