anv: do not export the Vulkan API

anv: do not export the Vulkan API

With version 1 of the Loader interface there is an internal/private symbol
(vk_icdGetInstanceProcAddr) which is used to retrieve all the API from the
Vulkan entrypoints from the ICD. Implying that exposing the Vulkan API is not
recommended.

Version 2 goes a step further explicitly forbiding the ICD from exposing Vulkan
symbols (and adding a negotiation API)

As a reference:
 - Nvidia 367.35
Missing negotiation API - version 1.
Exposes only vk_icdGetInstanceProcAddr.

 - AMD 16.30.3.306809
Have negotiation API - version 2,
Exposes vk_icdGetInstanceProcAddr.
Exposes a couple of Vulkan entry points - seems to be in violation with the spec.

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>