Fix: Build error with USD/Hydra with some build systems

Move API shared between USD and Hydra to usd_private.hh. It seems ninja
added an include directory other build systems don't.

source/blender/io/usd/hydra/world.cc

@@ -23,7 +23,6 @@
 
 #include "hydra_scene_delegate.hh"
 #include "image.hh"
-#include "usd_light_convert.hh"
 
 /* TODO: add custom `tftoken` "transparency"? */
 

source/blender/io/usd/intern/usd_light_convert.hh

@@ -9,8 +9,6 @@
 struct bNode;
 struct bNodeTree;
 
-struct Image;
-struct ImageUser;
 struct Main;
 struct Scene;
 
@@ -46,25 +44,4 @@ void dome_light_to_world_material(const USDImportParams &params,
                                   const pxr::UsdPrim &prim,
                                   const pxr::UsdTimeCode time = 0.0);
 
-/**
- * Helper struct for converting world shader nodes to a dome light, used by both
- * USD and Hydra. */
-struct WorldToDomeLight {
-  /* Image and its transform. */
-  Image *image = nullptr;
-  ImageUser *iuser = nullptr;
-  pxr::GfMatrix4d transform = pxr::GfMatrix4d(1.0);
-
-  /* Multiply image by color. */
-  bool mult_found = false;
-  float color_mult[4]{};
-
-  /* Fixed color. */
-  bool color_found = false;
-  float intensity = 0.0f;
-  float color[4]{};
-};
-
-void world_material_to_dome_light(const Scene *scene, WorldToDomeLight &res);
-
 }  // namespace blender::io::usd

source/blender/io/usd/usd_private.hh

@@ -2,6 +2,8 @@
  *
  * SPDX-License-Identifier: GPL-2.0-or-later */
 
+/* Functions shared between USD and Hydra, that are private to the USD module. */
+
 #pragma once
 
 #include <pxr/usd/usd/common.h>
@@ -11,6 +13,9 @@
 #include "usd.hh"
 
 struct Depsgraph;
+struct Image;
+struct ImageUser;
+struct Scene;
 
 namespace blender::io::usd {
 
@@ -22,4 +27,23 @@ std::string image_cache_file_path();
 std::string get_image_cache_file(const std::string &file_name, bool mkdir = true);
 std::string cache_image_color(const float color[4]);
 
+/** Result from converting world shader nodes to dome light parameters. */
+struct WorldToDomeLight {
+  /* Image and its transform. */
+  Image *image = nullptr;
+  ImageUser *iuser = nullptr;
+  pxr::GfMatrix4d transform = pxr::GfMatrix4d(1.0);
+
+  /* Multiply image by color. */
+  bool mult_found = false;
+  float color_mult[4]{};
+
+  /* Fixed color. */
+  bool color_found = false;
+  float intensity = 0.0f;
+  float color[4]{};
+};
+
+void world_material_to_dome_light(const Scene *scene, WorldToDomeLight &res);
+
 };  // namespace blender::io::usd