Update and rename vaadin-icons.scss to _vaadin-icons.scss (#9895)

Rename the vaadin-icons.scss to allow other libsass to compile the project successful.

themes/src/main/themes/VAADIN/themes/valo/fonts/vaadin-icons/vaadin-icons.scss → themes/src/main/themes/VAADIN/themes/valo/fonts/vaadin-icons/_vaadin-icons.scss

@@ -17,4 +17,4 @@ $vaadin-icons-pathPrefix: null;
 }
 
 @mixin vaadin-icons {
-}
+}