Dualcpu: Fix NS mailbox linking error

A quick workaround to fix NS mailbox linking error which is caused by
missing tfm_ns_mailbox_tx_stats_init.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I18562b6a793aaf919a0099ee6f45fbb7b5b4a918
2 files changed