Don't define bool for C23.

Upstream commit: https://sourceforge.net/p/xmlrpc-c/code/3318/

--- xmlrpc-c-1.60.05/lib/util/include/bool.h	2005-10-24 07:56:02.000000000 +0100
+++ linux-rv64/work/xmlrpc-c-1.60.05/lib/util/include/bool.h	2025-07-07 01:10:46.945707869 +0100
@@ -5,14 +5,3 @@
    collision.
 */
 
-#ifndef __cplusplus
-/* At least the GNU compiler defines __bool_true_false_are_defined */
-#ifndef __bool_true_false_are_defined
-#define __bool_true_false_are_defined
-typedef enum {
-    false = 0,
-    true = 1
-} bool;
-#endif
-#endif
-