um: Include generic barrier.h

...to get smp_store_release().
Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/include/asm/Kbuild

@@ -4,3 +4,4 @@ generic-y += ftrace.h pci.h io.h param.h delay.h mutex.h current.h exec.h
 generic-y += switch_to.h clkdev.h
 generic-y += trace_clock.h
 generic-y += preempt.h
+generic-y += barrier.h