patch 8.2.4038: various code not used when features are disabled

Problem:    Various code not used when features are disabled.
Solution:   Add #ifdefs. (Dominique Pellé, closes #9491)
diff --git a/src/edit.c b/src/edit.c
index 4d5ee85..739f078 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -1715,6 +1715,7 @@
     }
 }
 
+#if defined(FEAT_JOB_CHANNEL) || defined(PROTO)
 /*
  * Set the insert start position for when using a prompt buffer.
  */
@@ -1728,6 +1729,7 @@
     Insstart_blank_vcol = MAXCOL;
     arrow_used = FALSE;
 }
+#endif
 
 /*
  * Undo the previous edit_putchar().