Fix C23 build by renaming variable 'true'
Reported by gcc 13.2 with -std=gnu2x:
xprop.c: In function ‘Handle_Question_Mark’:
xprop.c:1291:10: error: expected identifier or ‘(’ before ‘true’
1291 | long true;
| ^~~~
xprop.c:1293:49: error: lvalue required as unary ‘&’ operand
1293 | dformat = Scan_Exp(dformat, thunks, format, &true);
| ^