BUG#52737: plugin_dir is set to /usr/local/mysql/lib/plugin while starting via mysqld_safe

Incremental patch to fix plugin_dir from .../lib/plugin to .../lib/mysql/plugin.

scripts/mysqld_safe.sh

@@ -704,7 +704,7 @@ fi
 
 cmd="`mysqld_ld_preload_text`$NOHUP_NICENESS"
 
-plugin_dir="${PLUGIN_DIR:-$MY_BASEDIR_VERSION/lib/plugin}${PLUGIN_VARIANT}"
+plugin_dir="${PLUGIN_DIR:-$MY_BASEDIR_VERSION/lib/mysql/plugin}${PLUGIN_VARIANT}"
 
 for i in  "$ledir/$MYSQLD" "$defaults" "--basedir=$MY_BASEDIR_VERSION" \
   "--datadir=$DATADIR" "--plugin-dir=$plugin_dir" "$USER_OPTION"