selinux: Add fcontext for core dumps

Previously core dumps could not be dumped, because the path was missing
the correct selinux context.
This patch updates the fcontext file to set the right context on the
path in question.

Change-Id: I000f5ff925d2962cc4042233c6c9a3bb2d2faada
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120065
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>