ox-icalendar skips the following event with a diary timestamp during
export:

  * First Sunday of the month
  <%%(diary-float t 0 1)>

ox-icalendar actually has longstanding code to handle diary timestamps
[1], but that code path is not reached, in part because
`org-export--skip-p' always skips timestamps with type `diary'.

The attached patch fixes this by including diary timestamps when the
export option `:with-timestamps' is `active'. I think it's reasonable
to consider diary timestamps as a type of active timestamp during
export, since they are included in the agenda, and have angle
brackets.

[1] https://git.savannah.gnu.org/cgit/emacs/org-mode.git/tree/lisp/ox-icalendar.el?id=07dd3bcae6b7b5e0692fc40dd307a7e841179b52#n826