patch 9.0.0090: no error when assigning bool to a string option

Problem:    No error when assigning bool to a string option with setwinvar().
Solution:   Give an error (closes #10766)

src/proto/option.pro

@@ -43,6 +43,7 @@ char_u *get_term_code(char_u *tname);
 char_u *get_highlight_default(void);
 char_u *get_encoding_default(void);
 int is_option_allocated(char *name);
+int is_string_option(char_u *name);
 int makeset(FILE *fd, int opt_flags, int local_only);
 int makefoldset(FILE *fd);
 void clear_termoptions(void);