patch 8.0.0846: cannot get the name of the pty of a job

Problem:    Cannot get the name of the pty of a job.
Solution:   Add the "tty" entry to the job info. (Ozaki Kiichi, closes #1920)
            Add the term_gettty() function.
diff --git a/src/proto/terminal.pro b/src/proto/terminal.pro
index 556f9dd..157b6aa 100644
--- a/src/proto/terminal.pro
+++ b/src/proto/terminal.pro
@@ -23,6 +23,7 @@
 void f_term_getsize(typval_T *argvars, typval_T *rettv);
 void f_term_getstatus(typval_T *argvars, typval_T *rettv);
 void f_term_gettitle(typval_T *argvars, typval_T *rettv);
+void f_term_gettty(typval_T *argvars, typval_T *rettv);
 void f_term_list(typval_T *argvars, typval_T *rettv);
 void f_term_scrape(typval_T *argvars, typval_T *rettv);
 void f_term_sendkeys(typval_T *argvars, typval_T *rettv);