MAINTAINERS: Add Eric Snowberg as a reviewer to IMA

Digital signature based IMA-appraisal relies heavily on kernel keyrings.
Eric Snowberg has been involved in adding the machine keyring to allow
the system owner to add their own keys.  With this addition,
IMA-appraisal usage can be extended to allow loading local and 3rd party
software keys onto the IMA keyring.

Add Eric as a reviewer.

Acked-by: Eric Snowberg <eric.snowberg@oracle.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>