MAINTAINERS: add missing security/integrity/platform_certs

Define a new KEYS/KEYRINGS_INTEGRITY record so that any changes
to platform_certs/ are posted on the linux-integrity mailing list
as well.

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>