fix: correctly set xkb_rules to allocated struct on init (#38)

Previously, the globals xkb_rules variable would
not be used, since it was set to NULL initally,
and was never updated via the config setter cb.

dscm/config.h

@@ -207,6 +207,7 @@ setter_xkb_rules(void *cvar, SCM value)
 		if (xkb->options) free((char*)xkb->options);
 	} else {
 		xkb = calloc(1, sizeof(struct xkb_rule_names));
+		(*((struct xkb_rule_names**)cvar)) = xkb;
 	}
 
 	xkb->rules = dscm_assoc_ref_string(value, "rules");