TS_SINGLE tab style has been deprecated in oxygen qt style,
which should be replaced by TS_PLAIN.

Gentoo-bug: 563802

--- a/rc/oxygenrc	2014-01-17 14:47:36.000000000 +0100
+++ b/rc/oxygenrc	2017-06-24 15:49:36.607616582 +0200
@@ -31,7 +31,7 @@
 ScrollBarBevel=true
 ProgressBarAnimated=true
 MenuHighlightMode=MM_DARK
-TabStyle=TS_SINGLE
+TabStyle=TS_PLAIN
 
 WindowDragEnabled=true
 WindowDragMode=WD_FULL