certs: Fix misaligned data in extra certificate list

Fix the following warning found by kbuild:

	certs/system_certificates.S:24: Error: misaligned data

because:

	KEYS: Reserve an extra certificate symbol for inserting without recompiling

doesn't correctly align system_extra_cert_used.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Mehmet Kayaalp <mkayaalp@linux.vnet.ibm.com>