um: Include generic barrier.h

...to get smp_store_release().

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
diff --git a/arch/um/include/asm/Kbuild b/arch/um/include/asm/Kbuild
index fdde187..fb3a6cc 100644
--- a/arch/um/include/asm/Kbuild
+++ b/arch/um/include/asm/Kbuild
@@ -4,3 +4,4 @@
 generic-y += switch_to.h clkdev.h
 generic-y += trace_clock.h
 generic-y += preempt.h
+generic-y += barrier.h