editors/cpeditor: add workaround for cmake 3.21.1

CMake Error:
  Running
   '/usr/local/bin/ninja' '-C' '/wrkdirs/usr/ports/editors/cpeditor/work/.build' '-t' 'recompact'
  failed with:
   ninja: error: build.ninja:824: multiple rules generate ui/ui_appwindow.h [-w dupbuild=err]

PR:		257489