patch 9.0.2025: no cmdline completion for ++opt args

Problem:  no cmdline completion for ++opt args
Solution: Add cmdline completion for :e ++opt=arg and :terminal
          [++options]

closes: #13319

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>
diff --git a/src/proto/optionstr.pro b/src/proto/optionstr.pro
index 88034fc..22601ba 100644
--- a/src/proto/optionstr.pro
+++ b/src/proto/optionstr.pro
@@ -189,6 +189,7 @@
 int expand_set_winaltkeys(optexpand_T *args, int *numMatches, char_u ***matches);
 int expand_set_wincolor(optexpand_T *args, int *numMatches, char_u ***matches);
 int check_ff_value(char_u *p);
+char_u *get_fileformat_name(expand_T *xp, int idx);
 void save_clear_shm_value(void);
 void restore_shm_value(void);
 /* vim: set ft=c : */