Fix bug#47923 New "mf_keycache.c" requires thread support

The bug is a compilation issue:
Function "find_key_block()" had thread operations
which were not guarded by "#if THREAD", add that now.

mysys/mf_keycache.c:
  Without thread-support, we are in a single-threaded world,
  so there is no parallelism, and the keycache cannot be
  within a resize operation while this request is being processed.

mysys/mf_keycache.c

@@ -1742,6 +1742,7 @@ restart:
       - block assigned but not yet read from file (invalid data).
   */
 
+#if THREAD
   if (keycache->in_resize)
   {
     /* This is a request during a resize operation */
@@ -1983,6 +1984,9 @@ restart:
     }
     DBUG_RETURN(0);
   }
+#else /* THREAD */
+  DBUG_ASSERT(!keycache->in_resize);
+#endif
 
   if (page_status == PAGE_READ &&
       (block->status & (BLOCK_IN_EVICTION | BLOCK_IN_SWITCH |