Fix scene index cmake build (#2631)

* add missing compile definition when building separate shared libraries with cmake

* address review issues

Author: cpichard

plugins/scene_index/CMakeLists.txt

@@ -49,6 +49,7 @@ if (BUILD_SCENE_INDEX_PLUGIN)
         if (MTOA_BUILD)
             target_compile_definitions(sceneIndexArnold PRIVATE MTOA_BUILD=1)
         endif()
+        target_compile_definitions(sceneIndexArnold PUBLIC ENABLE_SCENE_INDEX=1)
 
         if (BUILD_HEADERS_AS_SOURCES)
             target_sources(sceneIndexArnold PRIVATE ${HDR})