ManhattanStyle: Explicit PM_ButtonIconSize of 16

This fixes squeezed icons on some Linux Distros with QStyle
integrations, where a lower value would be set and the icons
be squeezed and become blurry.

Change-Id: I2962dd370491fc741646f52a9794e178a2ff81b1
Reviewed-by: hjk <hjk@theqtcompany.com>

src/plugins/coreplugin/manhattanstyle.cpp

@@ -199,6 +199,7 @@ int ManhattanStyle::pixelMetric(PixelMetric metric, const QStyleOption *option,
             retval = 1;
         break;
     case PM_ToolBarIconSize:
+    case PM_ButtonIconSize:
         if (panelWidget(widget))
             retval = 16;
         break;