patch 8.1.0027: difficult to make a plugin that feeds a line to a job

Problem:    Difficult to make a plugin that feeds a line to a job.
Solution:   Add the nitial code for the "prompt" buftype.
diff --git a/src/proto/channel.pro b/src/proto/channel.pro
index 8d26158..e6c9508 100644
--- a/src/proto/channel.pro
+++ b/src/proto/channel.pro
@@ -71,4 +71,5 @@
 void job_info(job_T *job, dict_T *dict);
 void job_info_all(list_T *l);
 int job_stop(job_T *job, typval_T *argvars, char *type);
+void invoke_prompt_callback(void);
 /* vim: set ft=c : */