CI: extend spellchecker whitelist, add "clen" as well

"clen" is all around the code, since codespell cannot distingush
variables names, let us ignore it
diff --git a/.github/workflows/codespell.yml b/.github/workflows/codespell.yml
index 1e118c7..bacd876 100644
--- a/.github/workflows/codespell.yml
+++ b/.github/workflows/codespell.yml
@@ -16,4 +16,4 @@
     - uses: codespell-project/actions-codespell@master
       with:
         skip: CHANGELOG,Makefile,*.fig,*.pem,./doc/design-thoughts,./doc/internals
-        ignore_words_list: ist,ists,hist,wan,ca,cas,que,ans,te,nd,referer,ot,uint,iif,fo,keep-alives,dosen,ifset,thrid,strack,ba,chck,hel,unx,mor
+        ignore_words_list: ist,ists,hist,wan,ca,cas,que,ans,te,nd,referer,ot,uint,iif,fo,keep-alives,dosen,ifset,thrid,strack,ba,chck,hel,unx,mor,clen