Cleanup: correct usage of extern-C blocks in various places

This removes extern-C blocks around other includes and adds
such blocks for some headers that need them.

source/blender/render/intern/include/renderpipeline.h

@@ -30,10 +30,18 @@ struct RenderData;
 struct RenderLayer;
 struct RenderResult;
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct RenderLayer *render_get_active_layer(struct Render *re, struct RenderResult *rr);
 void render_update_anim_renderdata(struct Render *re,
                                    struct RenderData *rd,
                                    struct ListBase *render_layers);
 void render_copy_renderdata(struct RenderData *to, struct RenderData *from);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* __RENDERPIPELINE_H__ */