ima: Use designated initializers for struct ima_event_data

Designated initializers allow specifying only the members of the struct
that need initialization. Non-mentioned members are initialized to zero.

This makes the code a bit clearer (particularly in ima_add_boot_aggregate)
and also allows adding a new member to the struct without having to update
all struct initializations.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>