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