auctex: add etoolbox dependency to fix failing build

pkgs/tools/typesetting/tex/auctex/default.nix

···
   buildInputs = [
     emacs
     ghostscript
-    (texliveBasic.withPackages (ps: [ ps.hypdoc ]))
+    (texliveBasic.withPackages (ps: [ ps.etoolbox ps.hypdoc ]))
   ];
 
   preConfigure = ''