Revert "Unconstify xkb_rules_names"

This reverts commit d007cd0a1f.

This is in fact more restrictive, because it breaks the (common) case
where the strings are const themselved, e.g. "evdev", "us", etc. As is
you must either duplicate the strings or suppress the warnings.

If the user needs to retain the non-const strings, he should instead
just keep them in some other struct and use xkb_rules_names just as
a temporary parameter for xkb_map_new_from_names. Mildly annoying but
acceptable.

include/xkbcommon/xkbcommon.h

@@ -111,11 +111,11 @@ typedef uint32_t xkb_led_index_t;
  * should be the primary identifier for a keymap.
  */
 struct xkb_rule_names {
-    char *rules;
+    const char *rules;
-    char *model;
+    const char *model;
-    char *layout;
+    const char *layout;
-    char *variant;
+    const char *variant;
-    char *options;
+    const char *options;
 };
 
 /**