patch 9.0.0689: compiler warning for unused function

Problem:    Compiler warning for unused function.
Solution:   Add #ifdef. (John Marriott)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 9f36dc9..468c384 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -2682,6 +2682,7 @@
     return r;
 }
 
+#ifdef HAS_MESSAGE_WINDOW
 /*
  * Get a count before a command.  Can only be a number.
  * Returns zero if there is no count.
@@ -2707,6 +2708,7 @@
     }
     return atol((char *)p);
 }
+#endif
 
 /*
  * Get the compilation type that should be used for "ufunc".