Qt: Fix when build with Qt < 4.8.0

StyleName is not available in Qt < 4.8

Change-Id: Icbfcef7c62b124ce67f1a02ed928a96233ae2d9d
Reviewed-on: https://code.wireshark.org/review/3508
Reviewed-by: Christopher Maynard <Christopher.Maynard@gtech.com>