nassh: dist: fix hterm.js building

Make sure to declare another libdot.js as external so rollup doesn't
duplicate it into the hterm dep.

Change-Id: If392dad597eb68df0d441338fd0d335dfbc86514
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/4852381
Reviewed-by: Joel Hockey <[email protected]>
Tested-by: kokoro <[email protected]>

Author: vapier

nassh/rollup.config.js

@@ -68,6 +68,7 @@ let targets = [
     external: [
       '../../../libdot/index.js',
       '../../libdot/index.js',
+      '../libdot/index.js',
     ],
     plugins: plugins,
   },