fix missing rebuild-threads option

Author: Thomas Bruckmann

pyxbackup

@@ -589,7 +589,7 @@ def _apply_log(bkp, incrdir=None, final=False):
     if xb_ibx_bin != 'innobackupex':
         ibx_opts = '--prepare '
         if XB_VERSION_MAJOR == 8:
-            ibx_opts = ' --rebuild-threads=%d' % xb_opt_rebuild_threads
+            ibx_opts += ' --rebuild-threads=%d' % xb_opt_rebuild_threads
     else: ibx_opts = '--apply-log '
 
     ibx_opts += " --use-memory=%dM" % xb_opt_prepare_memory