certs: add .gitignore to stop git nagging about x509_certificate_list

Currently we see this in "git status" if we build in the source dir:

Untracked files:
  (use "git add <file>..." to include in what will be committed)

        certs/x509_certificate_list

It looks like it used to live in kernel/ so we squash that .gitignore
entry at the same time.  I didn't bother to dig through git history to
see when it moved, since it is just a minor annoyance at most.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: keyrings@linux-nfs.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David Howells <dhowells@redhat.com>