<https://webkit.org/b/128452> Don't duplicate the list of input files for postprocess-headers.sh

Reviewed by Dan Bernstein.

* postprocess-headers.sh: Pull the list of headers to process out of the environment.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@163710 268f45cc-cd09-0410-ab3c-d52691b4dbfc