build: Update build.gn with AV1 headers

av1std_decode.h & av1std.h were added recently and needed adding to the
build.gn build.

Author: charles-lunarg

BUILD.gn

@@ -56,6 +56,8 @@ source_set("vulkan_headers") {
     "include/vulkan/vulkan.hpp",
     "include/vulkan/vulkan_core.h",
     "include/vulkan/vulkan_screen.h",
+    "include/vk_video/vulkan_video_codec_av1std_decode.h",
+    "include/vk_video/vulkan_video_codec_av1std.h",
     "include/vk_video/vulkan_video_codec_h264std_decode.h",
     "include/vk_video/vulkan_video_codec_h264std_encode.h",
     "include/vk_video/vulkan_video_codec_h264std.h",