patch 8.2.1662: :mksession does not restore shared terminal buffer properly

Problem:    :mksession does not restore shared terminal buffer properly.
Solution:   Keep a hashtab with terminal buffers. (Rob Pilling, closes #6930)
diff --git a/src/hashtab.c b/src/hashtab.c
index f114b28..dc0cbb6 100644
--- a/src/hashtab.c
+++ b/src/hashtab.c
@@ -81,7 +81,7 @@
 	vim_free(ht->ht_array);
 }
 
-#if defined(FEAT_SPELL) || defined(PROTO)
+#if defined(FEAT_SPELL) || defined(FEAT_TERMINAL) || defined(PROTO)
 /*
  * Free the array of a hash table and all the keys it contains.  The keys must
  * have been allocated.  "off" is the offset from the start of the allocate