[common] Add the definition of a legacy messaging interface

[common] Add the definition of a legacy messaging interface

This recent change that introduced new binary messaging interfaces,
replacing XW_MESSAGING_INTERFACE_1 with XW_MESSAGING_INTERFACE_2.
This chang broken the build of webapi-plugins-teec project, which depends
on the declaration of the legacy interface.
This commit fixes this problem, by adding the legacy interface back
to XW_Extension.h.

[Verification] webapi-plugins builds. webapi-plugins-teec builds,
when webapi-plugins built with this change is installed in GBS root.

Change-Id: Ia057a8e9ea9d51e06616a0ad16dc28b3ea342d8c