refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/sign_defs.h

@@ -34,7 +34,7 @@ struct sign_entry {
 
 /// Sign attributes. Used by the screen refresh routines.
 typedef struct {
-  char_u *text;
+  char *text;
   int hl_attr_id;
   int priority;
 } SignTextAttrs;