tst_QTextLayout: fix unhandled enum value in switch warning

Change-Id: I006286487a409d492fe398a2fed5dd854f758f9f
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>

tests/auto/gui/text/qtextlayout/tst_qtextlayout.cpp

@@ -1587,6 +1587,8 @@ void tst_QTextLayout::testTabDPIScale()
             case QPaintDevice::PdmPhysicalDpiX:
             case QPaintDevice::PdmPhysicalDpiY:
                 return 72;
+            case QPaintDevice::PdmDevicePixelRatio:
+                ; // fall through
             }
             return 0;
         }