hwcontext_vulkan: add VK_EXT_layer_settings to optional instance extensions

The issue is that some compilers complain if a struct or array
is empty.

This extension does nothing by default, and can be useful, so just add it
to keep the array non-empty.

libavutil/hwcontext_vulkan.c

@@ -582,6 +582,7 @@ typedef struct VulkanOptExtension {
 } VulkanOptExtension;
 
 static const VulkanOptExtension optional_instance_exts[] = {
+    { VK_EXT_LAYER_SETTINGS_EXTENSION_NAME,                   FF_VK_EXT_NO_FLAG                },
 #ifdef __APPLE__
     { VK_KHR_PORTABILITY_ENUMERATION_EXTENSION_NAME,          FF_VK_EXT_NO_FLAG                },
 #endif