keyboard: Add "print screen" and "scroll lock" to compose key options

These are reasonable enough that we don't have to hide them.

https://bugzilla.gnome.org/show_bug.cgi?id=733400

panels/keyboard/cc-keyboard-option.c

@@ -98,6 +98,8 @@ static const gchar *xkb_option_comp_whitelist[] = {
   "compose:lctrl",
   "compose:rctrl",
   "compose:caps",
+  "compose:prsc",
+  "compose:sclk",
   NULL
 };