patch 9.0.0379: cleaning up after writefile() is a hassle

Problem:    Cleaning up after writefile() is a hassle.
Solution:   Add the 'D' flag to defer deleting the written file.  Very useful
            in tests.
diff --git a/src/proto/userfunc.pro b/src/proto/userfunc.pro
index 3e417dd..074a2b8 100644
--- a/src/proto/userfunc.pro
+++ b/src/proto/userfunc.pro
@@ -58,6 +58,7 @@
 void func_ref(char_u *name);
 void func_ptr_ref(ufunc_T *fp);
 void ex_return(exarg_T *eap);
+int add_defer(char_u *name, int argcount_arg, typval_T *argvars);
 void handle_defer(void);
 void ex_call(exarg_T *eap);
 int do_return(exarg_T *eap, int reanimate, int is_cmd, void *rettv);