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/proto/terminal.pro b/src/proto/terminal.pro
index e8760d1..f3bd5a3 100644
--- a/src/proto/terminal.pro
+++ b/src/proto/terminal.pro
@@ -2,7 +2,7 @@
 void init_job_options(jobopt_T *opt);
 buf_T *term_start(typval_T *argvar, char **argv, jobopt_T *opt, int flags);
 void ex_terminal(exarg_T *eap);
-int term_write_session(FILE *fd, win_T *wp);
+int term_write_session(FILE *fd, win_T *wp, hashtab_T *terminal_bufs);
 int term_should_restore(buf_T *buf);
 void free_terminal(buf_T *buf);
 void free_unused_terminals(void);