Fix -Wbad-function-cast warnings from casting Atom to void *
xclipboard.c: In function ‘InsertClipboard’:
xclipboard.c:494:33: warning: cast from function call of type ‘Atom’ {aka ‘long unsigned int’} to non-matching type ‘void *’ [-Wbad-function-cast]
494 | (XtPointer)(XA_COMPOUND_TEXT(d)),
| ^
xclipboard.c: In function ‘LoseSelection’:
xclipboard.c:637:25: warning: cast from function call of type ‘Atom’ {aka ‘long unsigned int’} to non-matching type ‘void *’ [-Wbad-function-cast]
637 | (XtPointer)(XA_UTF8_STRING(d)), CurrentTime);
| ^