vim-patch:8.1.0939: no completion for sign group names

Problem:    No completion for sign group names.
Solution:   Add completion for sign group names and buffer names. (Yegappan
            Lakshmanan, closes vim/vim#3980)
3678f65d43

src/nvim/charset.c

@@ -1570,6 +1570,7 @@ char_u* skiptohex(char_u *q)
 ///
 /// @return Pointer to the next whitespace or NUL character.
 char_u *skiptowhite(const char_u *p)
+  FUNC_ATTR_NONNULL_ALL
 {
   while (*p != ' ' && *p != '\t' && *p != NUL) {
     p++;