refactor: replace char_u with char

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

src/nvim/ui.c

@@ -565,7 +565,7 @@ void ui_check_mouse(void)
   // - 'a' is in 'mouse' and "c" is in MOUSE_A, or
   // - the current buffer is a help file and 'h' is in 'mouse' and we are in a
   //   normal editing mode (not at hit-return message).
-  for (char_u *p = (char_u *)p_mouse; *p; p++) {
+  for (char *p = p_mouse; *p; p++) {
     switch (*p) {
     case 'a':
       if (vim_strchr(MOUSE_A, checkfor) != NULL) {