Change ${TARGET_NAME}_INCLUDE_DIR to PARENT_SCOPE.

cmake/external/protobuf.cmake

@@ -18,7 +18,7 @@ FUNCTION(build_protobuf TARGET_NAME BUILD_FOR_HOST)
     SET(PROTOBUF_SOURCES_DIR ${THIRD_PARTY_PATH}/${TARGET_NAME})
     SET(PROTOBUF_INSTALL_DIR ${THIRD_PARTY_PATH}/install/${TARGET_NAME})
 
-    SET(${TARGET_NAME}_INCLUDE_DIR "${PROTOBUF_INSTALL_DIR}/include")
+    SET(${TARGET_NAME}_INCLUDE_DIR "${PROTOBUF_INSTALL_DIR}/include" PARENT_SCOPE)
     SET(PROTOBUF_INCLUDE_DIR "${PROTOBUF_INSTALL_DIR}/include" PARENT_SCOPE)
     SET(${TARGET_NAME}_LITE_LIBRARY
         "${PROTOBUF_INSTALL_DIR}/lib/libprotobuf-lite${STATIC_LIBRARY_SUFFIX}"