um: Include generic barrier.h

...to get smp_store_release().

Change-Id: I74ee26375eccebd5f2cd10cf330690dfe7d5d949
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Git commit: 9af2452a
Git repo: https://github.com/torvalds/linux.git


Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>