fix clippy error

Author: xiaopengli89

wgpu-hal/src/vulkan/adapter.rs

@@ -1499,6 +1499,7 @@ impl super::Instance {
                 }),
             image_format_list: phd_capabilities.device_api_version >= vk::API_VERSION_1_2
                 || phd_capabilities.supports_extension(khr::image_format_list::NAME),
+            #[cfg(windows)]
             external_memory_win32: phd_capabilities.device_api_version >= vk::API_VERSION_1_1
                 || phd_capabilities.supports_extension(khr::external_memory_win32::NAME),
         };

wgpu-hal/src/vulkan/mod.rs

@@ -451,6 +451,7 @@ struct PrivateCapabilities {
     robust_image_access2: bool,
     zero_initialize_workgroup_memory: bool,
     image_format_list: bool,
+    #[cfg(windows)]
     external_memory_win32: bool,
 }