Add cast to clear -Wsign-compare warning

ico.c: In function ‘do_ico_window’:
ico.c:1084:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if (xev.xclient.data.l[0] == wm_delete_window)
                              ^~

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
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