Build: Add mailbox_common in the build

This relates to 'mailbox: Commonise mailbox_init' as it
introduces a common implementation for the tfm_ns_mailbox_init()
function.
The newly added common file is therefore added to the build.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I343bce7ef030d963bbf8822253f6ead849bb435f
1 file changed