The declaration of tui_set_var_cmd is not in sync with the definition.
Since tui_set_var_cmd is only used in the file where it's defined,
remove the declaration and make the definition static.
gdb/ChangeLog:
* tui-win.h (tui_set_var_cmd): Remove.
* tui-win.c (tui_set_var_cmd): Make static.
Change-Id: If4bddbfb573347fb7254fb6f1a940052a72f464f