seccomp: add mbind() to the syscall whitelist

seccomp: add mbind() to the syscall whitelist

The "memory-backend-ram" QOM object utilizes the mbind(2) syscall to
set the policy for a memory range.  Add the syscall to the seccomp
sandbox whitelist.

Signed-off-by: Paul Moore <pmoore@redhat.com>
Signed-off-by: Eduardo Otubo <eduardo.otubo@profitbricks.com>
Acked-by: Eduardo Otubo <eduardo.otubo@profitbricks.com>
Tested-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>