Fix the error path of PLUGIN_ROOT (#10446)

Summary:
When we try to use RocksDB with plugins as a third-party library for other databases, the plugin folder cannot be compiled correctly because of the wrong PLUGIN_ROOT variable. So we fix this error to ensure that it works perfectly when the directory of RocksDB is not the root directory.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10446

Reviewed By: jay-zhuang

Differential Revision: D38371321

Pulled By: ajkr

fbshipit-source-id: 0801b7b7dfa87751c8332fb52aac569dcdd72b5d

Co-authored-by: SuperMT <supertempler@gmail.com>

CMakeLists.txt

@@ -1009,7 +1009,7 @@ set(ROCKSDB_PLUGIN_BUILTINS "")
 message(STATUS "ROCKSDB PLUGINS TO BUILD ${ROCKSDB_PLUGINS}")
 list(APPEND PLUGINS ${ROCKSDB_PLUGINS})
 foreach(PLUGIN IN LISTS PLUGINS)
-  set(PLUGIN_ROOT "${CMAKE_SOURCE_DIR}/plugin/${PLUGIN}/")
+  set(PLUGIN_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/plugin/${PLUGIN}/")
   message("including rocksb plugin ${PLUGIN_ROOT}")
   set(PLUGINMKFILE "${PLUGIN_ROOT}${PLUGIN}.mk")
   if (NOT EXISTS ${PLUGINMKFILE})