tty: include kref.h in tty_driver.h

We use kref in tty_driver.h, but do not include kref.h. It is currently
included by linux/cdev.h -> linux/kobject.h -> linux/kref.h chain, so
everything is in order only implicitly. So make this dependency
explicit.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210723103147.18250-3-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/tty_driver.h

@@ -233,6 +233,7 @@
 
 #include <linux/export.h>
 #include <linux/fs.h>
+#include <linux/kref.h>
 #include <linux/list.h>
 #include <linux/cdev.h>
 #include <linux/termios.h>