Unconstify xkb_rules_names

Since we never return an xkb_rules_names and it's all user-provided
strings, seems a bit harsh to have it const.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>

include/xkbcommon/xkbcommon.h

@@ -114,11 +114,11 @@ typedef uint32_t xkb_led_index_t;
  * should be the primary identifier for a keymap.
  */
 struct xkb_rule_names {
-    const char *rules;
-    const char *model;
-    const char *layout;
-    const char *variant;
-    const char *options;
+    char *rules;
+    char *model;
+    char *layout;
+    char *variant;
+    char *options;
 };
 
 /**