ring-buffer: Select IRQ_WORK

As the wake up logic for waiters on the buffer has been moved
from the tracing code to the ring buffer, it requires also adding
IRQ_WORK as the wake up code is performed via irq_work.

This fixes compile breakage when a user of the ring buffer is selected
but tracing and irq_work are not.

Link http://lkml.kernel.org/r/20130503115332.GT8356@rric.localhost

Cc: Arnd Bergmann <arnd@arndb.de>
Reported-by: Robert Richter <rric@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/Kconfig

@@ -71,6 +71,7 @@ config TRACE_CLOCK
 config RING_BUFFER
 	bool
 	select TRACE_CLOCK
+	select IRQ_WORK
 
 config FTRACE_NMI_ENTER
        bool
@@ -107,7 +108,6 @@ config TRACING
 	select BINARY_PRINTF
 	select EVENT_TRACING
 	select TRACE_CLOCK
-	select IRQ_WORK
 
 config GENERIC_TRACER
 	bool