vulkan: update Vulkan headers to 1.1.68

Key changes from 1.1.61:
* Introduced split headers:
  * vulkan_core.h provides all declarations that don't depend on
    platform-specific headers
  * vulkan_android.h provides Android-specific declarations
  * vulkan.h is retained for source compatibility, and includes both
    of the above; vulkan_android.h is conditional on
* Re-defined symbols for extensions that were merged into Vk1.1 as
  typedefs/aliases for the new core symbols.
* New extensions:
  * VK_EXT_conservative_rasterization
  * VK_EXT_debug_utils
  * VK_EXT_external_memory_dma_buf
  * VK_EXT_external_memory_host
  * VK_EXT_global_priority
  * VK_EXT_queue_family_foreign
  * VK_AMD_shader_image_load_store_lod
  * VK_AMD_shader_info

Test: build
Change-Id: I84cdb4c24eed27ead99047951f795ccafa0bfc9a
7 files changed