Our rule right now is to use <> for external headers only.
util/sys_membarrier.c violates that. Fix it up.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Message-Id: <20180329151018.15319-1-brogers@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>