build: Re-add missing API declaration -include for EDS backend

Originally added in commit 7a9c8133468c1f8eb7fbb0e68a6a7e061b9452fb,
mistakenly removed in commit 9864169bfd0631ca5b46eccb77c37ea6411059a1.
Whoops. :-(

Bug reported on the Telepathy mailing list:
http://lists.freedesktop.org/archives/telepathy/2014-October/006635.html