Always add sitemap file to project files if sitemap is requested
* lisp/org-publish.el (org-publish-get-base-files): Add sitemap file.
I noticed some wonkiness in getting my sitemap created on my webserver
when pushing my website, and the problem seems to lie in
org-publish-get-base-files only returning existing files, and not
picking up on the soon to be generated sitemap. My patch always adds
the sitemap file to the list of returned files if a sitemap is
requested, regardless of if it exists or not.