[MacPorts] #44381: Add port:flex to graphviz build dependencies

[MacPorts]

#44381: Add port:flex to graphviz build dependencies
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  Reporter:  michaelld@…              |      Owner:  ryandesign@…
      Type:  defect                   |     Status:  new
  Priority:  Normal                   |  Milestone:
 Component:  ports                    |    Version:  2.3.1
Resolution:                           |   Keywords:
      Port:  graphviz graphviz-devel  |
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Comment (by ryandesign@…):

 I've no objection to that, we would just need to figure out which of the
 (sub)ports need that dependency.

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Ticket URL: <https://trac.macports.org/ticket/44381#comment:2>
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