gnome-usage: fix build with vala 0.42

srcpkgs/gnome-usage/patches/fix-vala-0-42-build.patch

@@ -0,0 +1,20 @@
+--- src/speedometer.vala
++++ src/speedometer.vala
+@@ -33,7 +33,7 @@ namespace Usage
+ 
+         private Gtk.CssProvider css_provider;
+ 
+-        private int _percentage;
++        private int _percentage = 0;
+         public int percentage {
+             get {
+                 return _percentage;
+@@ -43,7 +43,7 @@ namespace Usage
+ 
+                 _percentage = value;
+             }
+-            default = 0; }
++        }
+ 
+         construct {
+             css_provider = new Gtk.CssProvider();