printk: add dummy printk_force_console_enter/exit helpers

printk: add dummy printk_force_console_enter/exit helpers

The newly added interface is broken when PRINTK is disabled:

drivers/tty/sysrq.c: In function '__handle_sysrq':
drivers/tty/sysrq.c:601:9: error: implicit declaration of function 'printk_force_console_enter' [-Wimplicit-function-declaration]
  601 |         printk_force_console_enter();
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/tty/sysrq.c:611:25: error: implicit declaration of function 'printk_force_console_exit' [-Wimplicit-function-declaration]
  611 |                         printk_force_console_exit();
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~

Add empty stub functions for both.

Fixes: ed76c07c6885 ("printk: Introduce FORCE_CON flag")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Tested-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Link: https://lore.kernel.org/r/20241112142939.724093-1-arnd@kernel.org
Signed-off-by: Petr Mladek <pmladek@suse.com>