patch 8.2.2993: 'fileencodings' default value should depend on 'encoding'

Problem:    'fileencodings' default value should depend on 'encoding'. (Gary
            Johnson)
Solution:   When 'encoding' is "utf-8" use a different default value for
            'fileencodings'.
diff --git a/src/proto/option.pro b/src/proto/option.pro
index 8d672aa..25cfb65 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -1,5 +1,6 @@
 /* option.c */
 void set_init_1(int clean_arg);
+void set_fencs_unicode(void);
 void set_string_default(char *name, char_u *val);
 void set_number_default(char *name, long val);
 void set_local_options_default(win_T *wp, int do_buffer);