anv: fix VkExternalBufferProperties for host allocation

It was reported as unsupported previously.  It should be importable
and is compatible with itself.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Fixes: 69cc6272 ("anv: Implement VK_EXT_external_memory_host")
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit 58241303)