UI: Add missing previousIcon in Rachni theme

The previous icon for the media icons set in the Rachni theme is missing
in the theme's .qss file.

UI/data/themes/Rachni.qss

@@ -1471,6 +1471,10 @@ QPushButton#sourceFiltersButton {
     qproperty-icon: url(./Dark/media/media_next.svg);
 }
 
+* [themeID="previousIcon"] {
+    qproperty-icon: url(./Dark/media/media_previous.svg);
+}
+
 /* YouTube Integration */
 OBSYoutubeActions {
     qproperty-thumbPlaceholder: url(./Dark/sources/image.svg);