patch 8.2.2944: Vim9: no error when using job or channel as a string

Problem:    Vim9: no error when using job or channel as a string.
Solution:   Be more strict about conversion to string. (closes #8312)
diff --git a/src/proto/channel.pro b/src/proto/channel.pro
index bbe5288..32e68d8 100644
--- a/src/proto/channel.pro
+++ b/src/proto/channel.pro
@@ -58,4 +58,5 @@
 void f_ch_sendraw(typval_T *argvars, typval_T *rettv);
 void f_ch_setoptions(typval_T *argvars, typval_T *rettv);
 void f_ch_status(typval_T *argvars, typval_T *rettv);
+char_u *channel_to_string_buf(typval_T *varp, char_u *buf);
 /* vim: set ft=c : */
diff --git a/src/proto/job.pro b/src/proto/job.pro
index bf97577..3927e7f 100644
--- a/src/proto/job.pro
+++ b/src/proto/job.pro
@@ -34,4 +34,5 @@
 void f_job_start(typval_T *argvars, typval_T *rettv);
 void f_job_status(typval_T *argvars, typval_T *rettv);
 void f_job_stop(typval_T *argvars, typval_T *rettv);
+char_u *job_to_string_buf(typval_T *varp, char_u *buf);
 /* vim: set ft=c : */