Fix flatbuffer build against previous build changes

This fixes the flatc calls which were broken due to clearer separation
of build and source directory usage.

Author: pmai

CMakeLists.txt

@@ -111,9 +111,9 @@ if(OSI_BUILD_FLATBUFFER)
     set(fbs "${proto_base}.fbs")
     add_custom_command(
       OUTPUT "${fbs}"
-      COMMAND $<TARGET_FILE:flatc> -I "${PROTOBUF_IMPORT_DIRS}" -o "${CMAKE_CURRENT_BINARY_DIR}" --proto "${CMAKE_CURRENT_SOURCE_DIR}/${proto}"
+      COMMAND $<TARGET_FILE:flatc> -I "${PROTOBUF_IMPORT_DIRS}" -I "${CMAKE_CURRENT_BINARY_DIR}" -o "${CMAKE_CURRENT_BINARY_DIR}" --proto "${proto}"
       DEPENDS "${proto}" flatc
-      WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
+      WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
       COMMENT "Convert ${proto} to ${fbs} using flatc"
     )
     list(APPEND FLAT_FBS "${CMAKE_CURRENT_BINARY_DIR}/${fbs}")