annotate reftest.list for linux opt reftest failure, in test added earlier for Bug 477171. a=orange

layout/reftests/svg/filters/reftest.list

@@ -84,5 +84,5 @@ fails == filter-marked-line-01.svg pass.svg # bug 477704
 
 == feDiffuseLighting-1.svg feDiffuseLighting-1-ref.svg
 
-== feSpecularLighting-1.svg feSpecularLighting-1-ref.svg
+random-if(gtk2Widget) == feSpecularLighting-1.svg feSpecularLighting-1-ref.svg # bug 610945