mytharchive: fix use of reserved keywords for C23

Testing with Fedora rawhide, builds fail with:

    error: cannot use keyword 'false' as enumeration constant
    typedef enum { false = 0, true = 1 }    bool;
                   ^~~~~
    note: 'false' is a keyword with '-std=c23' onwards
    error: expected ';', identifier or '(' before 'bool'
    typedef enum { false = 0, true = 1 }    bool;
                                            ^~~~

mythplugins/mytharchive/mytharchivehelper/external/pxsup2dast.c

@@ -27,7 +27,9 @@
  #
  */
 
+#if __STDC_VERSION__ < 202311L // C23
 typedef enum { false = 0, true = 1 }	bool;
+#endif
 typedef unsigned char			bool8;