[MacPorts] #44381: graphviz, graphviz-devel: Add port:flex build dependency

[MacPorts]

#44381: graphviz, graphviz-devel: Add port:flex build dependency
--------------------------------------+------------------------
  Reporter:  michaelld                |      Owner:  ryandesign
      Type:  defect                   |     Status:  new
  Priority:  Normal                   |  Milestone:
 Component:  ports                    |    Version:  2.3.1
Resolution:                           |   Keywords:
      Port:  graphviz graphviz-devel  |
--------------------------------------+------------------------

Comment (by Smattr):

 For some further context, Graphviz ''did'' actually make some changes that
 briefly broke building with the default macOS Flex. But I believe these
 never made it into a release before we reverted them. The sad sob story:
 https://gitlab.com/graphviz/graphviz/-/commit/b64593afc07d3f62a13a3b6b634bcb80b7fe6180

-- 
Ticket URL: <https://trac.macports.org/ticket/44381#comment:5>
MacPorts <https://www.macports.org/>
Ports system for macOS