relay: select CONFIG_IRQ_WORK

The relay code was changed to use irq_work, and it now fails to build
if that is disabled:

kernel/relay.o: In function `__relay_reset':
relay.c:(.text.__relay_reset+0xb0): undefined reference to `irq_work_sync'
kernel/relay.o: In function `relay_close_buf':
relay.c:(.text.relay_close_buf+0x14): undefined reference to `irq_work_sync'
kernel/relay.o: In function `relay_switch_subbuf':

This adds the necessary Kconfig select statement.

Link: http://lkml.kernel.org/r/20160912154035.3222156-1-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: c37d492 ("relay: Use irq_work instead of plain timer for deferred wakeup")
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

init/Kconfig

@@ -1278,6 +1278,7 @@ config SYSFS_DEPRECATED_V2
 
 config RELAY
 	bool "Kernel->user space relay support (formerly relayfs)"
+	select IRQ_WORK
 	help
 	  This option enables support for relay interface support in
 	  certain file systems (such as debugfs).