s390: select CONFIG_TTY for use of tty in unconditional keyboard driver

The unconditionally built keyboard driver, drivers/s390/char/keyboard.c,
requires CONFIG_TTY, so select it from CONFIG_S390 to prevent a build
error.
Signed-off-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -140,6 +140,7 @@ config S390
 	select OLD_SIGACTION
 	select OLD_SIGSUSPEND3
 	select SYSCTL_EXCEPTION_TRACE
+	select TTY
 	select VIRT_CPU_ACCOUNTING
 	select VIRT_TO_BUS