sh: select IRQ_FORCED_THREADING.

With virtual IRQs fixed up with the genirq nothread infrastructure,
IRQ threads can be enabled across the board.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

arch/sh/Kconfig

@@ -21,6 +21,7 @@ config SUPERH
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_GENERIC_HARDIRQS
 	select HAVE_SPARSE_IRQ
+	select IRQ_FORCED_THREADING
 	select RTC_LIB
 	select GENERIC_ATOMIC64
 	select GENERIC_IRQ_SHOW