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/structs.h b/src/structs.h
index b70b00f..cbebd7f 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2356,6 +2356,11 @@
 
     int		b_shortname;	/* this file has an 8.3 file name */
 
+#ifdef FEAT_JOB_CHANNEL
+    char_u	*b_prompt_text;	     // set by prompt_setprompt()
+    char_u	*b_prompt_callback;  // set by prompt_setcallback()
+    partial_T	*b_prompt_partial;   // set by prompt_setcallback()
+#endif
 #ifdef FEAT_MZSCHEME
     void	*b_mzscheme_ref; /* The MzScheme reference to this buffer */
 #endif