scripts: add extract-cert and sign-file to .gitignore

...so "git status" doesn't nag us about them.

Cc: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>