refactor: replace char_u with char

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

src/nvim/syntax_defs.h

@@ -30,7 +30,7 @@ struct keyentry {
   int16_t *next_list;           // ID list for next match (if non-zero)
   int flags;
   int k_char;                   // conceal substitute character
-  char_u keyword[1];            // actually longer
+  char keyword[1];              // actually longer
 };
 
 // Struct used to store one state of the state stack.