patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)
diff --git a/src/proto/evalvars.pro b/src/proto/evalvars.pro
index 352097d..c53c115 100644
--- a/src/proto/evalvars.pro
+++ b/src/proto/evalvars.pro
@@ -13,7 +13,7 @@
 int get_spellword(list_T *list, char_u **pp);
 void prepare_vimvar(int idx, typval_T *save_tv);
 void restore_vimvar(int idx, typval_T *save_tv);
-list_T *heredoc_get(exarg_T *eap, char_u *cmd);
+list_T *heredoc_get(exarg_T *eap, char_u *cmd, int script_get);
 void ex_let(exarg_T *eap);
 void ex_const(exarg_T *eap);
 int ex_let_vars(char_u *arg_start, typval_T *tv, int copy, int semicolon, int var_count, int flags, char_u *op);